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Pre-build Symfony Caches in Docker Containers

In a typical Symfony project there are multiple caches built when running the cache:warmup command. By pre-building those caches and putting them into the container, the startup time can be reduced.


  1. Metadata Cache: Stores class metadata. ✅ Already in container
  2. Query Cache: Stores DQL queries converted to SQL. This is generated at runtime.
    💡Can be collected on CI -> Turned out to be too complex, instead I recommend to use a cache with 2 adapters: a local file cache backed by a shared external cache like Redis.
  3. Result Cache: Stores results of queries. ❌Only at runtime
  4. Proxy directory: Generated proxy objects which make lazy loading of entities possible. /var/www/app/var/cache/[env]/doctrine/orm/Proxies ✅ Already in container


  1. Route annotations: Parsed controller route annotations.
  2. Container: Service configuration and initialization. ⚠️ contains credentials when using parameters. Switch to environment variables instead.
  3. Translations: /var/www/app/var/cache/[env]/translations/
  4. Annotations: A huge file of all annotations /var/www/app/var/cache/[env]/annotations.php
  5. Routes: ⚠️ contains domain names /var/www/app/var/cache/[env]/url_generating_routes.php


  1. Template cache: Twig templates compiled to PHP files.

Use env variables instead of parameters

  1. Make a script that checks that all parameters are accessed with %env(parameter_name)% to not leak any parameters
  2. Replace parameters with environment variables in Website project
  3. Make a script that checks that no parameter values are included in the generated container

How to test:

  • Build a new image that includes cache warm-up while building (and removes it from the
  • Start the image with CACHE_WARMUP=0 and run a request or bin/console. Run docker diff [image-hash] to list the changed files since the start.
  • The output should show show no files in var/cache/[env]/ContainerXYZ and way less files compared to running the image with CACHE_WARMUP=1


Measurements of a web-server pod using PHP-FPM and a separate nginx container:

  • ~1m 44 seconds to boot new node (EC2 instance)
  • ~15 seconds image pull
  • ~20 seconds cache building and warmup
  • ~5 seconds nginx retry interval
  • 70 seconds initial readiness probe delay

Times before optimizations
Best time: 1 + 70s = 71 s
Worst time: 60 + 44 + 1 + 15 + 70 = 190 = 3m 10s

Reducing the initial readiness probe delay (best case):
1 + 25s = 26s reduction of 63,4% vs current best case

Pre-building cache (best case):
1 + 1s = 2s reduction of 97,1% vs current best case

Pre-building cache (worst case):
1m 44s + 15 + 1 + 1 = 2m 1s ~33% faster than current worst case


⚠️ Something is wrong with preload, unclear what.

Without preload:

hyperfine -r 1000 --warmup 10 "bash"
Time (mean ± σ): 68.9 ms ± 12.6 ms [User: 3.7 ms, System: 5.9 ms]
Range (min … max): 42.3 ms … 179.9 ms 1000 runs

With preload:

hyperfine -r 1000 --warmup 10 "bash"
Time (mean ± σ): 67.2 ms ± 32.9 ms [User: 3.5 ms, System: 5.5 ms]
Range (min … max): 57.5 ms … 1074.3 ms 1000 runs


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© 2021 by Adrian Philipp