Elixir New Added 10 September 2026

Ecto: connection not available and request was dropped from queue

Every connection in the pool was busy for longer than queue_target allowed, so DBConnection gave up rather than queueing indefinitely. Raising pool_size is the reflex and rarely the cure: the pool is usually held by a handful of slow queries, or by processes doing HTTP calls inside a transaction.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# config/runtime.exs
config :my_app, MyApp.Repo,
  pool_size: 10,
  queue_target: 50,        # milliseconds
  queue_interval: 1000,
  timeout: 15_000

# Find what is holding connections, in the database rather than the app
# psql: SELECT pid, state, now() - query_start AS age, left(query, 80)
#       FROM pg_stat_activity WHERE state <> 'idle' ORDER BY age DESC LIMIT 10;

# Log slow queries so the offender is obvious next time
config :my_app, MyApp.Repo, log: :debug, telemetry_prefix: [:my_app, :repo]

# Never do slow work inside a transaction: the connection is checked out
Repo.transaction(fn -> ... end)     # no HTTP calls, no sleeps in here

# Long running jobs deserve their own pool, not the web pool
config :my_app, MyApp.JobRepo, pool_size: 4

How to diagnose Elixir errors

Elixir errors are shaped by the actor model: a GenServer.call timeout does not mean the server crashed, it means the server was busy for longer than the caller was willing to wait. The right question is usually "what is that process doing?" rather than "why did the call fail?". Because supervisors restart failed processes automatically, transient errors can also hide in the logs while the system appears healthy.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Attach to a running node with iex --remsh and inspect the process: Process.info(pid, :message_queue_len). A growing mailbox means the process is the bottleneck.
Use :observer.start() to see the supervision tree, process memory and message queues visually.
Move long-running work out of handle_call. Use handle_cast, a Task, or a dedicated pool so the GenServer stays responsive.
Check restart intensity. A supervisor that exceeds max_restarts takes down its own supervisor, producing a cascade that looks like an unrelated failure at the top.
For compile errors in dependencies, run mix deps.compile --force and check that any required native toolchain (make, gcc, erlang headers) is installed.

Tools worth reaching for

:observer.start()
iex --remsh
Process.info/2
mix deps.tree
:recon

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

hexdocs.pm

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