[Changelog] Introducing service probes (#2988)

* [Changelog] Introducing service probes

* Fix linter

Author: peterschmidt85

docs/blog/posts/probes.md

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+---
+title: Introducing service probes
+date: 2025-08-14
+description: HTTP readiness probes for services, inspired by Kubernetes—safer rollouts and clear runtime visibility.
+slug: probes
+image: https://dstack.ai/static-assets/static-assets/images/dstack-service-probes.png
+categories:
+  - Changelog
+---
+
+# Introducing service probes
+
+`dstack` services are long-running workloads—most often inference endpoints and sometimes web apps—that run continuously on GPU or CPU instances. They can scale across replicas and support rolling deployments.
+
+This release adds HTTP probes inspired by Kubernetes readiness probes. Probes periodically call an endpoint on each replica (for example, `/health`) to confirm it responds as expected. The result gives clear visibility into startup progress and, during rolling deployments, ensures traffic only shifts to a replacement replica after all configured probes have proven ready.
+
+<img src="https://dstack.ai/static-assets/static-assets/images/dstack-service-probes.png" width="630"/>
+
+<!-- more -->
+
+When a service starts, replicas may need time to load models and initialize dependencies. Without probes, a replica is considered ready as soon as the container starts. With probes, readiness is based on real responses.
+
+Each probe sends an HTTP request to a configured endpoint at a set interval. A `2xx` response counts as success.
+
+## Configuration
+
+Probes can be set via the `probes` property in a service configuration:
+
+<div editor-title="service.dstack.yml">
+
+```yaml
+type: service
+name: llama31
+
+python: 3.12
+env:
+  - HF_TOKEN
+commands:
+  - uv pip install vllm
+  - |
+    vllm serve meta-llama/Meta-Llama-3.1-8B-Instruct \
+      --max-model-len 4096 \
+      --tensor-parallel-size $DSTACK_GPUS_NUM
+port: 8000
+model: meta-llama/Meta-Llama-3.1-8B-Instruct
+
+probes:
+  - type: http
+    url: /health
+    interval: 15s
+
+replicas: 2
+
+resources:
+  gpu: 24GB..48GB
+```
+</div>
+
+In this example, `dstack` sends a GET `/health` request every 15 seconds to each replica.
+
+## Probe status
+
+<div class="termy">
+
+```shell
+$ dstack ps --verbose
+
+ NAME                            BACKEND          STATUS   PROBES  SUBMITTED
+ llama31 deployment=1                             running          11 mins ago
+   replica=0 job=0 deployment=0  aws (us-west-2)  running  ✓       11 mins ago
+   replica=1 job=0 deployment=1  aws (us-west-2)  running  ×       1 min ago
+```
+
+</div>
+
+In `dstack ps --verbose`, a replica shows `×` if the last probe failed, `~` while probes are succeeding but the [`ready_after`](../../docs/reference/dstack.yml/service.md#ready_after) threshold is not yet reached, and `✓` once the last `ready_after` checks have succeeded. Probes run for each replica while it is `running`.
+
+## Advanced configuration
+
+Probes support custom HTTP methods, headers (with environment variable interpolation), request bodies, timeouts, and multiple checks in sequence. For example:
+
+```yaml
+env:
+  - PROBES_API_KEY
+probes:
+  - type: http
+    method: post
+    url: /check-health
+    headers:
+      - name: X-API-Key
+        value: ${{ env.PROBES_API_KEY }}
+      - name: Content-Type
+        value: application/json
+    body: '{"level": 2}'
+    timeout: 20s
+```
+
+Note: request bodies are not allowed with `GET` or `HEAD` methods.
+
+## Rolling deployments
+
+During a rolling deployment, `dstack` starts a replacement replica, waits for it to be `running` and to pass its probes, then retires the old replica. This preserves availability while large models warm up.
+
+Probes give you visibility about health of each replica. During rolling updates they gate traffic so new replicas receive requests after their checks pass.
+
+See [services](../../docs/concepts/services.md#probes) and the [reference](../../docs/reference/dstack.yml/service.md#probes) for all options.
+
+!!! info "What's next?"
+    1. Check [Quickstart](../../docs/quickstart.md)
+    2. Learn about [services](../../docs/concepts/services.md)
+    3. Join [Discord :material-arrow-top-right-thin:{ .external }](https://discord.gg/u8SmfwPpMd){:target="_blank"}