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Subscription deduplication, active subscriptions registry and limits#895

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Subscription deduplication, active subscriptions registry and limits#895
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@enisdenjo enisdenjo commented Apr 4, 2026 •
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This is a stacked PR based on #620.

This PR adds subscription deduplication at the router level - the same mechanism that already existed for queries now also works for subscriptions (both HTTP streaming and WebSocket). It also introduces a global limit on concurrent long-lived clients (WebSocket connections and HTTP streaming responses), and cleans up naming throughout the codebase to better distinguish the two separate kinds of subscription tracking that now coexist.

Two kinds of subscriptions

It is worth being explicit about this because the codebase now has two distinct subscription concepts and they should not be confused.

Inbound subscriptions (ActiveSubscriptions) are subscriptions from clients to the router. Each one represents a client that is currently connected and receiving events. This is the new registry introduced in this PR.

Outbound subscriptions (CallbackSubscriptions, WsSubgraphExecutor) are subscriptions the router opens toward subgraphs in the background, either over HTTP callback protocol or WebSocket. These are invisible to the client - they are the upstream data sources that feed events into the router. Previously these were also called "active subscriptions" which made things ambiguous. They are now consistently named with the callback_ prefix or the ws_ prefix to make clear they are subgraph-facing, not client-facing.

What changed

Subscriptions are now deduplicated

Subscriptions use the same fingerprint key as queries (method + path + selected headers + schema checksum + normalized operation hash + variables + extensions) and share the same in-flight map. The dedup mechanism is fundamentally different though because subscriptions are long-lived streams, not one-shot responses.

The first client (the leader) starts an upstream subscription and its events are pumped into a tokio::broadcast channel registered in the ActiveSubscriptions registry. Every subsequent client that arrives with the same fingerprint while that subscription is still active subscribes to the same broadcast channel instead of opening a new upstream connection. When the upstream finishes (or all listeners drop), the ProducerHandle drops, which removes the entry from both the registry and the in-flight map. The next client then starts a fresh upstream.

This means N clients subscribing to the same query with the same variables against the same schema version result in exactly one upstream subgraph connection.

Transport-agnostic subscriptions deduplication

Subscriptions deduplication is completely transport-agnostic. HTTP streams and WebSocket requests share the same fingerprint space and the same in-flight map. This works for both queries and subscriptions.

The key enabler is that WebSocket subscribe messages are processed through a synthetic request - when a subscribe WebSocket message arrives, a synthetic POST is assembled using the WebSocket path and the connection's headers (filtered by the dedupe header policy). This synthetic request goes through the exact same execute_planned_request path as a real HTTP request, producing the same fingerprint. As a result:

  • A subscription started over HTTP and an identical one over WebSocket deduplicate against each other and share a single upstream connection
  • A query sent over WebSocket deduplicates with the same query sent over HTTP

No special casing is needed for transport anywhere in the dedup logic.

One important nuance: accept and other transport-specific headers (connection: upgrade for WebSockets) are part of the fingerprint when the headers dedupe config is set to all (the default). Since HTTP streaming clients send accept: text/event-stream or accept: multipart/mixed while WebSocket clients do not send an accept header at all, those requests will produce different fingerprints and will not deduplicate against each other by default. To get true cross-transport deduplication - where a WebSocket subscription and an SSE subscription with the same operation share one upstream connection and the events fan out to both - configure headers: none or explicitly exclude the transport-specific headers from the key:

traffic_shaping:
  router:
    dedupe:
      enabled: true
      headers: none

or with an explicit include list that omits transport-specific headers:

traffic_shaping:
  router:
    dedupe:
      enabled: true
      headers:
        include:
          - authorization
          - x-tenant-id

Single execute_planned_request for all transports

Before this PR, the WebSocket handler had its own copy of the entire execution pipeline - JWT validation, variable coercion, client request details assembly, plan execution, usage reporting - all duplicated from the HTTP handler. This was the main reason WebSocket subscriptions could not participate in deduplication.

Now both HTTP and WebSocket handlers call a single execute_planned_request function. The WebSocket handler constructs a synthetic Method, Uri, and HeaderMap from the WebSocket connection context and passes those in. The function has no knowledge of the transport - it just sees a method, URL, and headers, same as if it came from HTTP. This eliminates the duplication and is what makes transport-agnostic dedup possible.

The caller-owned InFlightCleanupGuard

Inflight map gains a new method get_or_try_init_with_guard where the guard is handed to the init closure as an argument. It is used only for subscriptions, and there - it is moved into the upstream pump task and lives as long as the stream, so joiners can find the entry and connect to the broadcast channel.

Why not change get_or_try_init?

Passing the guard into init also for queries and dropping it when init resolves introduced a 2x performance regression. I just couldn't figure out why. If you know, please share (browse code | see 2x regression).

No clones in InFlightMap

The inflight map's get_or_try_init previously cloned key and map unconditionally on every call (both leader and joiner paths) so they could be captured by the closure. By destructuring self upfront into its three fields, all three can be moved independently - cell directly into the get_or_try_init call, key and map directly into the guard inside the closure - eliminating all clones on this path.

Active subscriptions registry

A new ActiveSubscriptions registry sits in shared router state. Every active upstream subscription is registered there. The registry holds the broadcast sender for each subscription identified by a ULID. ProducerHandle is a RAII wrapper: when it drops, it removes the entry from the registry and drops the in-flight cleanup guard.

The registry is also the mechanism for graceful shutdown on schema reload: when a new supergraph is loaded, all active subscriptions are closed with a SUBSCRIPTION_SCHEMA_RELOAD error before the schema is swapped in. Clients receive this as a final error event and are expected to reconnect. This is the same error code Apollo Router uses.

SharedRouterResponse is now an enum

Before this PR SharedRouterResponse only represented a single buffered response body. Streaming responses bypassed it entirely and were returned as a separate PlannedResponse::Direct variant (from #620) that could not participate in the dedup path.

Now SharedRouterResponse is an enum with two variants:

  • Single - a buffered body with status code and headers
  • Stream - a broadcast sender, the pre-subscribed leader receiver, the stream content type, and headers

This unification means the whole pipeline collapses to a single execute_planned_request call for all transports and both operation types. The PlannedResponse enum is gone. Error count, usage reporting, and metric writing all work through the enum regardless of response type.

WebSocket subscription loop is now spawned

Previously the WebSocket frame handler drove the subscription event loop inline using "select" to race the upstream BoxStream against a cancel channel. This worked because the frame handler owned the stream directly - it could poll it and still break out on cancel.

Now the upstream is no longer held by the frame handler. execute_planned_request spawns a pump task that drains the upstream into a broadcast channel before returning. By the time the frame handler gets back a SharedRouterResponse::Stream it only has a broadcast receiver, not the stream itself. If the subscription loop then runs inline, the frame handler blocks on receiver.recv() and never reads another frame from the client - so a ClientMessage::Complete sent by the client sits unread in the WebSocket buffer forever and cancellation is broken.

The fix is to spawn the subscription loop as a separate task. The frame handler returns immediately and remains free to process incoming frames. When it eventually reads a Complete, it sends on the cancel channel which the spawned loop is selecting on. ServerMessage::complete is sent from inside the spawned task once the loop exits.

Long-lived client limit

A new middleware counts active WebSocket and HTTP streaming connections and rejects new ones with 503 Service Unavailable + Retry-After: 5 once the limit is reached. It is entirely branch-free on the hot path for non-streaming requests: the enabled flag is resolved once at app construction, and the streaming check uses cheap header lookups with a fast substring pre-filter before falling back to full Accept header parsing.

The limit defaults to 128 and only activates when at least one of WebSocket or Subscriptions is enabled and the limit is greater than zero. Configurable via traffic_shaping.router.max_long_lived_clients:

traffic_shaping:
  router:
    max_long_lived_clients: 256

Naming cleanup

The HTTP callback subscription infrastructure was renamed throughout to include the word callback so it is clearly distinct from the new inbound active subscriptions registry:

  • ActiveSubscription -> CallbackSubscription
  • ActiveSubscriptionsMap -> CallbackSubscriptionsMap
  • active_callback_subscriptions field -> callback_subscriptions
  • HeartbeatEnforcerTask -> CallbackHeartbeatEnforcerTask

ID generation switched to ULID

Subscription IDs in both the active subscriptions registry and the HTTP callback executor now use ULIDs instead of UUID v4. The HTTP callback protocol does not mandate any particular ID format - the spec only requires that the subscription ID and verifier are strings agreed upon between the router and the subgraph, so we are free to use whatever we want. ULID was chosen because it is extremely fast to generate.

Broadcast capacity is configurable

The broadcast channel buffer size is configurable via subscriptions.broadcast_capacity (default 32). When a consumer falls behind and the buffer fills up, it skips missed messages and continues from the latest available event - there is no error, just a trace log. This is intentional: the alternative (blocking the pump) would stall all other consumers, or dropping the client would be destructive (maybe it recovers).

subscriptions:
  enabled: true
  broadcast_capacity: 64

The internal mpsc buffers between the subgraph executor and the pump task are reduced from 256 to 16 now that back-pressure is handled by the broadcast channel's own buffer.

Open questions

Do we actually want cross-transport deduplication? The mechanism supports it, but it means a WebSocket client and an SSE client end up sharing the same broadcast channel and receiving the same raw event bytes. The bytes are valid for both because subscription events are just JSON, but it is worth deciding whether this is intentional behavior or an accidental side effect of the unified pipeline.

Should single responses deduplicate across transports too? The body is always JSON so it fits either transport. The awkward part is the status code and the content-type headers. For example, if one client accepts application/json and another application/graphql-response+json - they must get matching content types respectivelly. This might be an issue when the user excludes accept from deduplication - but its rare to have many parallel different transports - so its up for debate.

Gotchas

Late joiners do not get replayed events. A client that joins an already-running subscription receives events from the moment it subscribes onward. There is no replay of events already delivered to earlier clients. The promotion e2e test explicitly asserts this behavior.

The leader receiver is pre-subscribed before the pump spawns. The pump is spawned after the broadcast channel is created, but the leader's receiver is taken before the spawn. This closes the window where the channel could have zero receivers between spawn and the first consumer subscribing, which would cause events to be silently dropped by tokio's broadcast channel.

No outbound subscriptions deduplication. Often enough users emit an initial event on subscribe allowing clients to sync their state on subscribe. If we were to deduplicate outbound subscriptions (those going to subgraphs), clients would miss the initial events and their state could go out of sync. Maybe we can have an option? No, because subgraph teams are separate to gateway teams, and tracking down why an initial event is not propagated is time wasted.

TODOs

  • Performance regression (Read "Why not change get_or_try_init?" section)
  • Test max_long_lived_clients limits
  • Test supergraph schema reload kickoff
  • Can we buffer 1 the subgraph executor of http callbacks and websockets because the active subscriptions broadcaster handles backpressure?

enisdenjo and others added 30 commits April 3, 2026 17:14
@enisdenjo
this is ok except for websocket server hella ugly
de76583
@enisdenjo
of course I didnt run tests
9b787a6
@enisdenjo
dedupe what can be deduped
6d64f8e
@enisdenjo
haha
00dbae2
@enisdenjo
broadcast capacity
8377ac6
@enisdenjo
haha
d1e3160
@enisdenjo
he he
85e528d
@enisdenjo
format nad long lived client limit
ced2195
@enisdenjo
of course race condition
4e732d3
@enisdenjo
ok attempt to clean this up but lets rebase first
6f8e441
@enisdenjo
ok
2622dc8
@enisdenjo
remove unused
efccf7c
@enisdenjo
begin super unification the great joiner
7403f60
@enisdenjo
switch to ulid
4a44797 
fast
@enisdenjo
the great enforcer 2
d2192df
@enisdenjo
excuse me please work its still great
6923ad4
@enisdenjo
close the gap
0ff7156
@enisdenjo
WIP
c898f7d
@enisdenjo
now we're talking, the great divide
7cbbc15
@enisdenjo
lets talk naming
4fbaa84
@enisdenjo
thats right keep that thang simple
b428344
@enisdenjo
promotion
37f4a28
@enisdenjo
WS server dedupe start hehe
6c2d847
@enisdenjo
ok this looks good right
7e684a5
@enisdenjo
lol of course
c5764b3
@enisdenjo
right, needs a spawn
436a276
@enisdenjo
WS dedupe hehe
50734f0
@enisdenjo
big man thing across transports
ec9f98a
@enisdenjo
dedupe across boundaries fixed
91ff2dd
@enisdenjo
leave a comment
12b97a5
@enisdenjo enisdenjo marked this pull request as ready for review April 4, 2026 00:21
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Code Review

This pull request introduces a robust mechanism for subscription deduplication and concurrent connection limiting. It implements an ActiveSubscriptions registry that allows multiple clients with identical fingerprints to share a single upstream subscription via broadcast channels, supporting cross-transport deduplication between WebSockets and HTTP streams. A new LongLivedClientLimitService middleware is added to enforce a maximum number of concurrent streaming connections. Additionally, the PR refactors the execution pipeline to unify response handling and ensures active subscriptions are gracefully terminated during schema reloads. Feedback highlights a critical bug in the connection limit middleware where the client counter is decremented prematurely for streaming responses; the guard must be attached to the response extensions to persist for the duration of the connection.

Comment thread bin/router/src/pipeline/long_lived_client_limit.rs
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github-actions Bot commented Apr 4, 2026 •
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          "body": "This is a stacked PR based on #620.\r\n\r\nThis PR adds subscription deduplication at the router level - the same mechanism that already existed for queries now also works for subscriptions (both HTTP streaming and WebSocket). It also introduces a global limit on concurrent long-lived clients (WebSocket connections and HTTP streaming responses), and cleans up naming throughout the codebase to better distinguish the two separate kinds of subscription tracking that now coexist.\r\n\r\n# Two kinds of subscriptions\r\n\r\nIt is worth being explicit about this because the codebase now has two distinct subscription concepts and they should not be confused.\r\n\r\n**Inbound subscriptions** (`ActiveSubscriptions`) are subscriptions from clients to the router. Each one represents a client that is currently connected and receiving events. This is the new registry introduced in this PR.\r\n\r\n**Outbound subscriptions** (`CallbackSubscriptions`, `WsSubgraphExecutor`) are subscriptions the router opens toward subgraphs in the background, either over HTTP callback protocol or WebSocket. These are invisible to the client - they are the upstream data sources that feed events into the router. Previously these were also called \"active subscriptions\" which made things ambiguous. They are now consistently named with the `callback_` prefix or the `ws_` prefix to make clear they are subgraph-facing, not client-facing.\r\n\r\n# What changed\r\n\r\n## Subscriptions are now deduplicated\r\n\r\nSubscriptions use the same fingerprint key as queries (method + path + selected headers + schema checksum + normalized operation hash + variables + extensions) and share the same in-flight map. The dedup mechanism is fundamentally different though because subscriptions are long-lived streams, not one-shot responses.\r\n\r\nThe first client (the leader) starts an upstream subscription and its events are pumped into a `tokio::broadcast` channel registered in the `ActiveSubscriptions` registry. Every subsequent client that arrives with the same fingerprint while that subscription is still active subscribes to the same broadcast channel instead of opening a new upstream connection. When the upstream finishes (or all listeners drop), the `ProducerHandle` drops, which removes the entry from both the registry and the in-flight map. The next client then starts a fresh upstream.\r\n\r\nThis means N clients subscribing to the same query with the same variables against the same schema version result in exactly one upstream subgraph connection.\r\n\r\n## Transport-agnostic subscriptions deduplication\r\n\r\nSubscriptions deduplication is completely transport-agnostic. HTTP streams and WebSocket requests share the same fingerprint space and the same in-flight map. This works for both queries and subscriptions.\r\n\r\nThe key enabler is that WebSocket subscribe messages are processed through a synthetic request - when a subscribe WebSocket message arrives, a synthetic `POST` is assembled using the WebSocket path and the connection's headers (filtered by the dedupe header policy). This synthetic request goes through the exact same `execute_planned_request` path as a real HTTP request, producing the same fingerprint. As a result:\r\n\r\n- A subscription started over HTTP and an identical one over WebSocket deduplicate against each other and share a single upstream connection\r\n- A query sent over WebSocket deduplicates with the same query sent over HTTP\r\n\r\nNo special casing is needed for transport anywhere in the dedup logic.\r\n\r\nOne important nuance: `accept` and other transport-specific headers (`connection: upgrade` for WebSockets) are part of the fingerprint when the `headers` dedupe config is set to `all` (the default). Since HTTP streaming clients send `accept: text/event-stream` or `accept: multipart/mixed` while WebSocket clients do not send an `accept` header at all, those requests will produce different fingerprints and will not deduplicate against each other by default. To get true cross-transport deduplication - where a WebSocket subscription and an SSE subscription with the same operation share one upstream connection and the events fan out to both - configure `headers: none` or explicitly exclude the transport-specific headers from the key:\r\n\r\n```yaml\r\ntraffic_shaping:\r\n  router:\r\n    dedupe:\r\n      enabled: true\r\n      headers: none\r\n```\r\n\r\nor with an explicit include list that omits transport-specific headers:\r\n\r\n```yaml\r\ntraffic_shaping:\r\n  router:\r\n    dedupe:\r\n      enabled: true\r\n      headers:\r\n        include:\r\n          - authorization\r\n          - x-tenant-id\r\n```\r\n\r\n## Single `execute_planned_request` for all transports\r\n\r\nBefore this PR, the WebSocket handler had its own copy of the entire execution pipeline - JWT validation, variable coercion, client request details assembly, plan execution, usage reporting - all duplicated from the HTTP handler. This was the main reason WebSocket subscriptions could not participate in deduplication.\r\n\r\nNow both HTTP and WebSocket handlers call a single `execute_planned_request` function. The WebSocket handler constructs a synthetic `Method`, `Uri`, and `HeaderMap` from the WebSocket connection context and passes those in. The function has no knowledge of the transport - it just sees a method, URL, and headers, same as if it came from HTTP. This eliminates the duplication and is what makes transport-agnostic dedup possible.\r\n\r\n## The caller-owned `InFlightCleanupGuard`\r\n\r\nInflight map gains a new method `get_or_try_init_with_guard` where the guard is handed to the init closure as an argument. It is used only for subscriptions, and there - it is moved into the upstream pump task and lives as long as the stream, so joiners can find the entry and connect to the broadcast channel.\r\n\r\n### Why not change `get_or_try_init`?\r\n\r\nPassing the guard into init also for queries and dropping it when init resolves introduced a 2x performance regression. I just couldn't figure out why. If you know, please share ([browse code](https://github.com/graphql-hive/router/tree/0329884fadc3391d764d73e1a3d9a4f8b4aa67c3) | [see 2x regression](https://github.com/graphql-hive/router/actions/runs/23985118763/job/69955716903)).\r\n\r\n### No clones in `InFlightMap`\r\n\r\nThe inflight map's `get_or_try_init` previously cloned `key` and `map` unconditionally on every call (both leader and joiner paths) so they could be captured by the closure. By destructuring self upfront into its three fields, all three can be moved independently - `cell` directly into the `get_or_try_init` call, `key` and `map` directly into the guard inside the closure - eliminating all clones on this path.\r\n\r\n## Active subscriptions registry\r\n\r\nA new `ActiveSubscriptions` registry sits in shared router state. Every active upstream subscription is registered there. The registry holds the broadcast sender for each subscription identified by a [ULID](https://github.com/ulid/spec). `ProducerHandle` is a RAII wrapper: when it drops, it removes the entry from the registry and drops the in-flight cleanup guard.\r\n\r\nThe registry is also the mechanism for graceful shutdown on schema reload: when a new supergraph is loaded, all active subscriptions are closed with a `SUBSCRIPTION_SCHEMA_RELOAD` error before the schema is swapped in. Clients receive this as a final error event and are expected to reconnect. This is the same error code Apollo Router uses.\r\n\r\n## `SharedRouterResponse` is now an enum\r\n\r\nBefore this PR `SharedRouterResponse` only represented a single buffered response body. Streaming responses bypassed it entirely and were returned as a separate `PlannedResponse::Direct` variant (from #620) that could not participate in the dedup path.\r\n\r\nNow `SharedRouterResponse` is an enum with two variants:\r\n\r\n- `Single` - a buffered body with status code and headers\r\n- `Stream` - a broadcast sender, the pre-subscribed leader receiver, the stream content type, and headers\r\n\r\nThis unification means the whole pipeline collapses to a single `execute_planned_request` call for all transports and both operation types. The `PlannedResponse` enum is gone. Error count, usage reporting, and metric writing all work through the enum regardless of response type.\r\n\r\n## WebSocket subscription loop is now spawned\r\n\r\nPreviously the WebSocket frame handler drove the subscription event loop inline using \"select\" to race the upstream `BoxStream` against a cancel channel. This worked because the frame handler owned the stream directly - it could poll it and still break out on cancel.\r\n\r\nNow the upstream is no longer held by the frame handler. `execute_planned_request` spawns a pump task that drains the upstream into a broadcast channel before returning. By the time the frame handler gets back a `SharedRouterResponse::Stream` it only has a broadcast receiver, not the stream itself. If the subscription loop then runs inline, the frame handler blocks on `receiver.recv()` and never reads another frame from the client - so a `ClientMessage::Complete` sent by the client sits unread in the WebSocket buffer forever and cancellation is broken.\r\n\r\nThe fix is to spawn the subscription loop as a separate task. The frame handler returns immediately and remains free to process incoming frames. When it eventually reads a `Complete`, it sends on the cancel channel which the spawned loop is selecting on. `ServerMessage::complete` is sent from inside the spawned task once the loop exits.\r\n\r\n## Long-lived client limit\r\n\r\nA new middleware counts active WebSocket and HTTP streaming connections and rejects new ones with `503 Service Unavailable` + `Retry-After: 5` once the limit is reached. It is entirely branch-free on the hot path for non-streaming requests: the `enabled` flag is resolved once at app construction, and the streaming check uses cheap header lookups with a fast substring pre-filter before falling back to full Accept header parsing.\r\n\r\nThe limit defaults to 128 and only activates when at least one of WebSocket or Subscriptions is enabled and the limit is greater than zero. Configurable via `traffic_shaping.router.max_long_lived_clients`:\r\n\r\n```yaml\r\ntraffic_shaping:\r\n  router:\r\n    max_long_lived_clients: 256\r\n```\r\n\r\n## Naming cleanup\r\n\r\nThe HTTP callback subscription infrastructure was renamed throughout to include the word `callback` so it is clearly distinct from the new inbound active subscriptions registry:\r\n\r\n- `ActiveSubscription` -> `CallbackSubscription`\r\n- `ActiveSubscriptionsMap` -> `CallbackSubscriptionsMap`\r\n- `active_callback_subscriptions` field -> `callback_subscriptions`\r\n- `HeartbeatEnforcerTask` -> `CallbackHeartbeatEnforcerTask`\r\n\r\n## ID generation switched to ULID\r\n\r\nSubscription IDs in both the active subscriptions registry and the HTTP callback executor now use ULIDs instead of UUID v4. The HTTP callback protocol does not mandate any particular ID format - the spec only requires that the subscription ID and verifier are strings agreed upon between the router and the subgraph, so we are free to use whatever we want. ULID was chosen because it is [extremely fast to generate](https://github.com/dylanhart/ulid-rs).\r\n\r\n## Broadcast capacity is configurable\r\n\r\nThe broadcast channel buffer size is configurable via `subscriptions.broadcast_capacity` (default 32). When a consumer falls behind and the buffer fills up, it skips missed messages and continues from the latest available event - there is no error, just a trace log. This is intentional: the alternative (blocking the pump) would stall all other consumers, or dropping the client would be destructive (maybe it recovers).\r\n\r\n```yaml\r\nsubscriptions:\r\n  enabled: true\r\n  broadcast_capacity: 64\r\n```\r\n\r\nThe internal mpsc buffers between the subgraph executor and the pump task are reduced from 256 to 16 now that back-pressure is handled by the broadcast channel's own buffer.\r\n\r\n# Open questions\r\n\r\n**Do we actually want cross-transport deduplication?** The mechanism supports it, but it means a WebSocket client and an SSE client end up sharing the same broadcast channel and receiving the same raw event bytes. The bytes are valid for both because subscription events are just JSON, but it is worth deciding whether this is intentional behavior or an accidental side effect of the unified pipeline.\r\n\r\n**Should single responses deduplicate across transports too?** The body is always JSON so it fits either transport. The awkward part is the status code and the `content-type` headers. For example, if one client accepts `application/json` and another `application/graphql-response+json` - they must get matching content types respectivelly. This might be an issue when the user excludes `accept` from deduplication - but its rare to have many parallel different transports - so its up for debate.\r\n\r\n# Gotchas\r\n\r\n**Late joiners do not get replayed events.** A client that joins an already-running subscription receives events from the moment it subscribes onward. There is no replay of events already delivered to earlier clients. The promotion e2e test explicitly asserts this behavior.\r\n\r\n**The leader receiver is pre-subscribed before the pump spawns.** The pump is spawned after the broadcast channel is created, but the leader's receiver is taken before the spawn. This closes the window where the channel could have zero receivers between spawn and the first consumer subscribing, which would cause events to be silently dropped by tokio's broadcast channel.\r\n\r\n**No outbound subscriptions deduplication.** Often enough users emit an initial event on subscribe allowing clients to sync their state on subscribe. If we were to deduplicate outbound subscriptions (those going to subgraphs), clients would miss the initial events and their state could go out of sync. Maybe we can have an option? No, because subgraph teams are separate to gateway teams, and tracking down why an initial event is not propagated is time wasted.\r\n\r\n# TODOs\r\n\r\n- [x] Performance regression (Read \"Why not change get_or_try_init?\" section)\r\n- [x] Test max_long_lived_clients limits\r\n- [ ] Test supergraph schema reload kickoff\r\n- [ ] Can we buffer 1 the subgraph executor of http callbacks and websockets because the active subscriptions broadcaster handles backpressure?\r\n- [ ] Document\r\n",
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          "body": "This is a stacked PR based on #620.\r\n\r\nThis PR adds subscription deduplication at the router level - the same mechanism that already existed for queries now also works for subscriptions (both HTTP streaming and WebSocket). It also introduces a global limit on concurrent long-lived clients (WebSocket connections and HTTP streaming responses), and cleans up naming throughout the codebase to better distinguish the two separate kinds of subscription tracking that now coexist.\r\n\r\n# Two kinds of subscriptions\r\n\r\nIt is worth being explicit about this because the codebase now has two distinct subscription concepts and they should not be confused.\r\n\r\n**Inbound subscriptions** (`ActiveSubscriptions`) are subscriptions from clients to the router. Each one represents a client that is currently connected and receiving events. This is the new registry introduced in this PR.\r\n\r\n**Outbound subscriptions** (`CallbackSubscriptions`, `WsSubgraphExecutor`) are subscriptions the router opens toward subgraphs in the background, either over HTTP callback protocol or WebSocket. These are invisible to the client - they are the upstream data sources that feed events into the router. Previously these were also called \"active subscriptions\" which made things ambiguous. They are now consistently named with the `callback_` prefix or the `ws_` prefix to make clear they are subgraph-facing, not client-facing.\r\n\r\n# What changed\r\n\r\n## Subscriptions are now deduplicated\r\n\r\nSubscriptions use the same fingerprint key as queries (method + path + selected headers + schema checksum + normalized operation hash + variables + extensions) and share the same in-flight map. The dedup mechanism is fundamentally different though because subscriptions are long-lived streams, not one-shot responses.\r\n\r\nThe first client (the leader) starts an upstream subscription and its events are pumped into a `tokio::broadcast` channel registered in the `ActiveSubscriptions` registry. Every subsequent client that arrives with the same fingerprint while that subscription is still active subscribes to the same broadcast channel instead of opening a new upstream connection. When the upstream finishes (or all listeners drop), the `ProducerHandle` drops, which removes the entry from both the registry and the in-flight map. The next client then starts a fresh upstream.\r\n\r\nThis means N clients subscribing to the same query with the same variables against the same schema version result in exactly one upstream subgraph connection.\r\n\r\n## Transport-agnostic subscriptions deduplication\r\n\r\nSubscriptions deduplication is completely transport-agnostic. HTTP streams and WebSocket requests share the same fingerprint space and the same in-flight map. This works for both queries and subscriptions.\r\n\r\nThe key enabler is that WebSocket subscribe messages are processed through a synthetic request - when a subscribe WebSocket message arrives, a synthetic `POST` is assembled using the WebSocket path and the connection's headers (filtered by the dedupe header policy). This synthetic request goes through the exact same `execute_planned_request` path as a real HTTP request, producing the same fingerprint. As a result:\r\n\r\n- A subscription started over HTTP and an identical one over WebSocket deduplicate against each other and share a single upstream connection\r\n- A query sent over WebSocket deduplicates with the same query sent over HTTP\r\n\r\nNo special casing is needed for transport anywhere in the dedup logic.\r\n\r\nOne important nuance: `accept` and other transport-specific headers (`connection: upgrade` for WebSockets) are part of the fingerprint when the `headers` dedupe config is set to `all` (the default). Since HTTP streaming clients send `accept: text/event-stream` or `accept: multipart/mixed` while WebSocket clients do not send an `accept` header at all, those requests will produce different fingerprints and will not deduplicate against each other by default. To get true cross-transport deduplication - where a WebSocket subscription and an SSE subscription with the same operation share one upstream connection and the events fan out to both - configure `headers: none` or explicitly exclude the transport-specific headers from the key:\r\n\r\n```yaml\r\ntraffic_shaping:\r\n  router:\r\n    dedupe:\r\n      enabled: true\r\n      headers: none\r\n```\r\n\r\nor with an explicit include list that omits transport-specific headers:\r\n\r\n```yaml\r\ntraffic_shaping:\r\n  router:\r\n    dedupe:\r\n      enabled: true\r\n      headers:\r\n        include:\r\n          - authorization\r\n          - x-tenant-id\r\n```\r\n\r\n## Single `execute_planned_request` for all transports\r\n\r\nBefore this PR, the WebSocket handler had its own copy of the entire execution pipeline - JWT validation, variable coercion, client request details assembly, plan execution, usage reporting - all duplicated from the HTTP handler. This was the main reason WebSocket subscriptions could not participate in deduplication.\r\n\r\nNow both HTTP and WebSocket handlers call a single `execute_planned_request` function. The WebSocket handler constructs a synthetic `Method`, `Uri`, and `HeaderMap` from the WebSocket connection context and passes those in. The function has no knowledge of the transport - it just sees a method, URL, and headers, same as if it came from HTTP. This eliminates the duplication and is what makes transport-agnostic dedup possible.\r\n\r\n## The caller-owned `InFlightCleanupGuard`\r\n\r\nInflight map gains a new method `get_or_try_init_with_guard` where the guard is handed to the init closure as an argument. It is used only for subscriptions, and there - it is moved into the upstream pump task and lives as long as the stream, so joiners can find the entry and connect to the broadcast channel.\r\n\r\n### Why not change `get_or_try_init`?\r\n\r\nPassing the guard into init also for queries and dropping it when init resolves introduced a 2x performance regression. I just couldn't figure out why. If you know, please share ([browse code](https://github.com/graphql-hive/router/tree/0329884fadc3391d764d73e1a3d9a4f8b4aa67c3) | [see 2x regression](https://github.com/graphql-hive/router/actions/runs/23985118763/job/69955716903)).\r\n\r\n### No clones in `InFlightMap`\r\n\r\nThe inflight map's `get_or_try_init` previously cloned `key` and `map` unconditionally on every call (both leader and joiner paths) so they could be captured by the closure. By destructuring self upfront into its three fields, all three can be moved independently - `cell` directly into the `get_or_try_init` call, `key` and `map` directly into the guard inside the closure - eliminating all clones on this path.\r\n\r\n## Active subscriptions registry\r\n\r\nA new `ActiveSubscriptions` registry sits in shared router state. Every active upstream subscription is registered there. The registry holds the broadcast sender for each subscription identified by a [ULID](https://github.com/ulid/spec). `ProducerHandle` is a RAII wrapper: when it drops, it removes the entry from the registry and drops the in-flight cleanup guard.\r\n\r\nThe registry is also the mechanism for graceful shutdown on schema reload: when a new supergraph is loaded, all active subscriptions are closed with a `SUBSCRIPTION_SCHEMA_RELOAD` error before the schema is swapped in. Clients receive this as a final error event and are expected to reconnect. This is the same error code Apollo Router uses.\r\n\r\n## `SharedRouterResponse` is now an enum\r\n\r\nBefore this PR `SharedRouterResponse` only represented a single buffered response body. Streaming responses bypassed it entirely and were returned as a separate `PlannedResponse::Direct` variant (from #620) that could not participate in the dedup path.\r\n\r\nNow `SharedRouterResponse` is an enum with two variants:\r\n\r\n- `Single` - a buffered body with status code and headers\r\n- `Stream` - a broadcast sender, the pre-subscribed leader receiver, the stream content type, and headers\r\n\r\nThis unification means the whole pipeline collapses to a single `execute_planned_request` call for all transports and both operation types. The `PlannedResponse` enum is gone. Error count, usage reporting, and metric writing all work through the enum regardless of response type.\r\n\r\n## WebSocket subscription loop is now spawned\r\n\r\nPreviously the WebSocket frame handler drove the subscription event loop inline using \"select\" to race the upstream `BoxStream` against a cancel channel. This worked because the frame handler owned the stream directly - it could poll it and still break out on cancel.\r\n\r\nNow the upstream is no longer held by the frame handler. `execute_planned_request` spawns a pump task that drains the upstream into a broadcast channel before returning. By the time the frame handler gets back a `SharedRouterResponse::Stream` it only has a broadcast receiver, not the stream itself. If the subscription loop then runs inline, the frame handler blocks on `receiver.recv()` and never reads another frame from the client - so a `ClientMessage::Complete` sent by the client sits unread in the WebSocket buffer forever and cancellation is broken.\r\n\r\nThe fix is to spawn the subscription loop as a separate task. The frame handler returns immediately and remains free to process incoming frames. When it eventually reads a `Complete`, it sends on the cancel channel which the spawned loop is selecting on. `ServerMessage::complete` is sent from inside the spawned task once the loop exits.\r\n\r\n## Long-lived client limit\r\n\r\nA new middleware counts active WebSocket and HTTP streaming connections and rejects new ones with `503 Service Unavailable` + `Retry-After: 5` once the limit is reached. It is entirely branch-free on the hot path for non-streaming requests: the `enabled` flag is resolved once at app construction, and the streaming check uses cheap header lookups with a fast substring pre-filter before falling back to full Accept header parsing.\r\n\r\nThe limit defaults to 128 and only activates when at least one of WebSocket or Subscriptions is enabled and the limit is greater than zero. Configurable via `traffic_shaping.router.max_long_lived_clients`:\r\n\r\n```yaml\r\ntraffic_shaping:\r\n  router:\r\n    max_long_lived_clients: 256\r\n```\r\n\r\n## Naming cleanup\r\n\r\nThe HTTP callback subscription infrastructure was renamed throughout to include the word `callback` so it is clearly distinct from the new inbound active subscriptions registry:\r\n\r\n- `ActiveSubscription` -> `CallbackSubscription`\r\n- `ActiveSubscriptionsMap` -> `CallbackSubscriptionsMap`\r\n- `active_callback_subscriptions` field -> `callback_subscriptions`\r\n- `HeartbeatEnforcerTask` -> `CallbackHeartbeatEnforcerTask`\r\n\r\n## ID generation switched to ULID\r\n\r\nSubscription IDs in both the active subscriptions registry and the HTTP callback executor now use ULIDs instead of UUID v4. The HTTP callback protocol does not mandate any particular ID format - the spec only requires that the subscription ID and verifier are strings agreed upon between the router and the subgraph, so we are free to use whatever we want. ULID was chosen because it is [extremely fast to generate](https://github.com/dylanhart/ulid-rs).\r\n\r\n## Broadcast capacity is configurable\r\n\r\nThe broadcast channel buffer size is configurable via `subscriptions.broadcast_capacity` (default 32). When a consumer falls behind and the buffer fills up, it skips missed messages and continues from the latest available event - there is no error, just a trace log. This is intentional: the alternative (blocking the pump) would stall all other consumers, or dropping the client would be destructive (maybe it recovers).\r\n\r\n```yaml\r\nsubscriptions:\r\n  enabled: true\r\n  broadcast_capacity: 64\r\n```\r\n\r\nThe internal mpsc buffers between the subgraph executor and the pump task are reduced from 256 to 16 now that back-pressure is handled by the broadcast channel's own buffer.\r\n\r\n# Open questions\r\n\r\n**Do we actually want cross-transport deduplication?** The mechanism supports it, but it means a WebSocket client and an SSE client end up sharing the same broadcast channel and receiving the same raw event bytes. The bytes are valid for both because subscription events are just JSON, but it is worth deciding whether this is intentional behavior or an accidental side effect of the unified pipeline.\r\n\r\n**Should single responses deduplicate across transports too?** The body is always JSON so it fits either transport. The awkward part is the status code and the `content-type` headers. For example, if one client accepts `application/json` and another `application/graphql-response+json` - they must get matching content types respectivelly. This might be an issue when the user excludes `accept` from deduplication - but its rare to have many parallel different transports - so its up for debate.\r\n\r\n# Gotchas\r\n\r\n**Late joiners do not get replayed events.** A client that joins an already-running subscription receives events from the moment it subscribes onward. There is no replay of events already delivered to earlier clients. The promotion e2e test explicitly asserts this behavior.\r\n\r\n**The leader receiver is pre-subscribed before the pump spawns.** The pump is spawned after the broadcast channel is created, but the leader's receiver is taken before the spawn. This closes the window where the channel could have zero receivers between spawn and the first consumer subscribing, which would cause events to be silently dropped by tokio's broadcast channel.\r\n\r\n**No outbound subscriptions deduplication.** Often enough users emit an initial event on subscribe allowing clients to sync their state on subscribe. If we were to deduplicate outbound subscriptions (those going to subgraphs), clients would miss the initial events and their state could go out of sync. Maybe we can have an option? No, because subgraph teams are separate to gateway teams, and tracking down why an initial event is not propagated is time wasted.\r\n\r\n# TODOs\r\n\r\n- [x] Performance regression (Read \"Why not change get_or_try_init?\" section)\r\n- [x] Test max_long_lived_clients limits\r\n- [ ] Test supergraph schema reload kickoff\r\n- [ ] Can we buffer 1 the subgraph executor of http callbacks and websockets because the active subscriptions broadcaster handles backpressure?\r\n- [ ] Document\r\n",
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@enisdenjo enisdenjo marked this pull request as draft April 4, 2026 11:14
@enisdenjo enisdenjo marked this pull request as ready for review April 4, 2026 20:41
@enisdenjo enisdenjo merged commit 26f87ca into not-kamil-subs Apr 13, 2026
28 of 29 checks passed
@enisdenjo enisdenjo deleted the not-kamil-subs-single-subs branch April 13, 2026 11:40
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@dotansimha dotansimha dotansimha approved these changes

@ardatan ardatan ardatan approved these changes

@kamilkisiela kamilkisiela Awaiting requested review from kamilkisiela

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