Absolutely — here’s a clear walkthrough of what each service in your Docker Compose file does, why it’s there, and a few important fixes/recommendations. High-level overview (Prefect 3.x) - postgres: Stores Prefect’s state and metadata (flows, runs, blocks, etc.). - redis: Powers Prefect’s messaging and cache layers. - prefect-server: Runs Prefect’s web UI and API. - prefect-worker: Despite the name, this is not a “worker”; it starts Prefect’s background services (scheduler, late-run detection, etc.) separate from the web server. - prefect-worker-process: This is the actual worker process that polls a work pool and executes flow runs. Service-by-service explanation - postgres - Image: Postgres 14 - Environment: Creates a

prefect

DB with user/password

prefect

- Volume: Persists DB data across container restarts - Healthcheck: Uses

pg_isready

to ensure the DB is ready before other services start - redis - Image: Redis 7 - Volume: Persists Redis data - Healthcheck: Uses

redis-cli ping

to ensure Redis is responding - prefect-server - Image:

prefecthq/prefect:3-latest

- Depends on: Waits until Postgres and Redis are healthy - Environment: - `PREFECT_API_DATABASE_CONNECTION_URL`: Tells Prefect Server how to connect to Postgres (async SQLAlchemy URL) - `PREFECT_SERVER_API_HOST: 0.0.0.0`: Binds the API/UI to all interfaces in the container -

PREFECT_MESSAGING_*

and `PREFECT_REDIS_*`: Configure Prefect’s messaging to use your Redis service - Command:

prefect server start --no-services --analytics-off

- Runs only the web server (UI + API). The actual services (scheduler, etc.) are started in a separate container (see next service). - Ports:

4200:4200

exposes the UI and API at http://localhost:4200 - prefect-worker - Image:

prefecthq/prefect:3-latest

- Despite the name, this is the “server services” runner - Depends on: Postgres/Redis healthy - Environment: Same DB + Redis vars as the server (the services need these) - Command:

prefect server services start

- Starts all enabled Prefect services (scheduler, late runs, etc.) in one process - Verified via

prefect server services start --help

- prefect-worker-process - Image:

prefecthq/prefect:3-latest

- This is the actual worker that executes flows - Depends on:

prefect-server

started (so it can reach the API) - Environment: - `PREFECT_API_URL`: Points the worker to your Prefect API (the internal service name

prefect-server

is correct inside the compose network) - Command (needs a small fix; see below): - You have:

prefect worker start --pool worker-pool-example --type process

- Correct:

prefect worker start --pool worker-pool-example

- The

--type

flag is not valid on

prefect worker start

in Prefect 3.x. The worker type is defined on the work pool itself when you create it. - Verified with

prefect worker start --help

Important fixes and recommendations 1) Remove angle brackets in environment values - In your compose, you have: -

PREFECT_API_DATABASE_CONNECTION_URL: <postgresql+asyncpg://prefect:prefect@postgres:5432/prefect>

-

PREFECT_API_URL: <http://prefect-server:4200/api>

- Those angle brackets are placeholders — if left as-is, Prefect will read them literally and fail. - Use: -

PREFECT_API_DATABASE_CONNECTION_URL: <postgresql+asyncpg://prefect:prefect@postgres:5432/prefect>

-

PREFECT_API_URL: <http://prefect-server:4200/api>

2) Fix the worker start command - Create the work pool once (outside of compose) with the type: -

prefect work-pool create "worker-pool-example" --type process

- Then start your worker with: -

prefect worker start --pool worker-pool-example

- We validated the CLI: -

prefect worker start

requires

--pool

and does not accept

--type

-

prefect work-pool create

is where

--type

is used 3) Consider renaming services for clarity

-

prefect-worker

→

prefect-services

(it runs the server services, not a worker) -

prefect-worker-process

→

prefect-worker

(this is the actual worker) 4) Optional: add a healthcheck to prefect-server - Right now,

prefect-worker-process

waits for

service_started

, not “healthy”. - You can add a healthcheck to Prefect Server so the worker waits for the API to be live: - Example:

healthcheck:
      test: ["CMD-SHELL", "curl -sf <http://localhost:4200/api/health> || exit 1"]
      interval: 5s
      timeout: 5s
      retries: 10

- Then change the worker’s

depends_on

to

condition: service_healthy

Cleaned-up compose with the fixes applied - This keeps your split “webserver” and “services” pattern, removes angle brackets, and fixes the worker command.

services:
  postgres:
    image: postgres:14
    environment:
      POSTGRES_USER: prefect
      POSTGRES_PASSWORD: prefect
      POSTGRES_DB: prefect
    volumes:
      - postgres_data:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U prefect"]
      interval: 5s
      timeout: 5s
      retries: 5

  redis:
    image: redis:7
    volumes:
      - redis_data:/data
    healthcheck:
      test: ["CMD-SHELL", "redis-cli ping"]
      interval: 5s
      timeout: 5s
      retries: 5

  prefect-server:
    image: prefecthq/prefect:3-latest
    depends_on:
      postgres:
        condition: service_healthy
      redis:
        condition: service_healthy
    environment:
      PREFECT_API_DATABASE_CONNECTION_URL: <postgresql+asyncpg://prefect:prefect@postgres:5432/prefect>
      PREFECT_SERVER_API_HOST: 0.0.0.0
      PREFECT_MESSAGING_BROKER: prefect_redis.messaging
      PREFECT_MESSAGING_CACHE: prefect_redis.messaging
      PREFECT_REDIS_MESSAGING_HOST: redis
      PREFECT_REDIS_MESSAGING_PORT: 6379
      PREFECT_REDIS_MESSAGING_DB: 0
    command: prefect server start --no-services --analytics-off
    ports:
      - "4200:4200"
    # Optional: add a healthcheck so workers wait for a healthy API
    # healthcheck:
    #   test: ["CMD-SHELL", "curl -sf <http://localhost:4200/api/health> || exit 1"]
    #   interval: 5s
    #   timeout: 5s
    #   retries: 10

  prefect-services:
    image: prefecthq/prefect:3-latest
    depends_on:
      postgres:
        condition: service_healthy
      redis:
        condition: service_healthy
    environment:
      PREFECT_API_DATABASE_CONNECTION_URL: <postgresql+asyncpg://prefect:prefect@postgres:5432/prefect>
      PREFECT_MESSAGING_BROKER: prefect_redis.messaging
      PREFECT_MESSAGING_CACHE: prefect_redis.messaging
      PREFECT_REDIS_MESSAGING_HOST: redis
      PREFECT_REDIS_MESSAGING_PORT: 6379
      PREFECT_REDIS_MESSAGING_DB: 0
    command: prefect server services start

  prefect-worker:
    image: prefecthq/prefect:3-latest
    depends_on:
      prefect-server:
        condition: service_started # change to service_healthy if you add the healthcheck
    environment:
      PREFECT_API_URL: <http://prefect-server:4200/api>
    command: prefect worker start --pool worker-pool-example

volumes:
  postgres_data:
  redis_data:

How to bring this up and run flows 1) Start the stack: -

docker compose up -d

- UI will be at http://localhost:4200 2) Create the work pool (one time): -

prefect work-pool create "worker-pool-example" --type process

3) Start your worker (compose already does this). It will poll the pool and execute runs. Notes - The

PREFECT_MESSAGING_*

settings are configuring Prefect’s messaging provider to use Redis. The host/port/db point to your Redis service. - For production, change your Postgres credentials, consider secrets management, and ensure you persist volumes and secure the UI/API behind your network policies or a reverse proxy. - If you prefer a simpler setup, you can run everything in a single container with

prefect server start

(without

--no-services

) and remove the

prefect-services

container.

If you share how you plan to deploy (local dev vs. production) I can tailor this further and help with pool creation, TLS, or adding a reverse proxy. Relevant docs: Self-hosting Prefect and Workers and Work Pools.

Great question — and quick clarifier: when you say “fn vs non fn”, do you mean: - Prefect Functions (Cloud) vs. traditional worker-based runs, or - Running a local Prefect server with/without external services (Postgres/Redis)? Here’s a concise breakdown of what runs locally “without Compose” in the common cases: Local server without Docker Compose - Command:

prefect server start

- Components used by default: - Database: SQLite (a local file under your Prefect home directory); no Postgres needed - Messaging/Cache: In-memory; no Redis needed - Services: Background services (scheduler, late-run detection, etc.) run in the same process by default - UI/API: http://127.0.0.1:4200 - Notes: - Add

--no-services

if you only want the web UI/API and plan to run services separately (less common locally) - You can still override to use Postgres/Redis by setting env vars like

PREFECT_API_DATABASE_CONNECTION_URL

or

PREFECT_MESSAGING_*

Run a worker locally against Prefect Cloud (no local server) - You don’t need any local Postgres/Redis or a local server - Set your API to Cloud and start a worker:

prefect config set PREFECT_API_URL=<https://api.prefect.cloud/api/accounts/<acct>/workspaces/<ws>>
  prefect work-pool create my-pool --type process  # one-time
  prefect worker start --pool my-pool

- The worker polls Prefect Cloud and executes flows locally Self-hosted with external services (still without Compose) - If you do want Postgres/Redis without Compose, you can point Prefect at them:

export PREFECT_API_DATABASE_CONNECTION_URL=<postgresql+asyncpg://user:pass@host:5432/db>
  export PREFECT_MESSAGING_BROKER=prefect_redis.messaging
  export PREFECT_MESSAGING_CACHE=prefect_redis.messaging
  export PREFECT_REDIS_MESSAGING_HOST=redis-host
  export PREFECT_REDIS_MESSAGING_PORT=6379
  prefect server start

- This mirrors what your Compose file did but with your own managed services What runs by default (summary) - Database: SQLite (local file) - Messaging: in-memory - Cache: in-memory - Services: enabled, run in-process with the server unless you use

--no-services

Relevant docs for deeper detail - Server CLI: prefect server CLI - Settings (DB and messaging): Prefect settings reference - Workers and pools: Workers and Work Pools - Self-hosting overview: Self-hosting Prefect If by “fn” you meant Prefect Functions (Cloud), those do not use your local Postgres/Redis either — they run on Prefect’s managed infrastructure. Let me know which “fn” you meant and I’ll tailor the specifics.

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