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> They advertise themselves into etcd, and any dependencies are pulled from etcd. That means that the Django app gets the addresses for the PostgreSQL and Redis

by jorlow 12y ago

> They advertise themselves into etcd, and any dependencies are pulled from etcd. That means that the Django app gets the addresses for the PostgreSQL and Redis servers from etcd, and connects that way. If these values change, each container restarts itself as needed. Could you explain a bit more how that works?

HorizonXP 12y ago
Take Postgres for example. I use the official Docker image, and run it via a fleet .service file, having it store its data in a Docker data-only container, also launched from a fleet unit file. Finally, there's a third unit file that runs a slightly customized version of the docker-register image from CoreOS's polvi. This Dockerized app polls the Docker socket every 10s or so to get the IP and port information for the Docker container it's monitoring, in this case Postgres. It PUTs this value into etcd. Afterwards, when my Django containers start, they pull their configuration from etcd using python-etcd. It will get the IPs and ports for the services, including Postgres, from etcd, and configure itself using that. Finally, it will keep these values in a dictionary. If these values ever differ from what's currently in etcd, then my Django app will send a signal to cause uwsgi to gracefully restart the Django app, so that it can reload itself with the new configuration. For other Dockerized apps like nginx, I use kelseyhightower's confd-watch to monitor the etcd keys I'm interested in. confd-watch is great because it will use golang's text/template package to allow me to generate configuration files, like nginx.conf, with the values from etcd. So as these values get updated, so will nginx as confd-watch will force a graceful reload. Hope that helps.
- leonidlm 12y ago
  "If these values ever differ from what's currently in etcd, then my Django app will send a signal to cause uwsgi to gracefully restart the Django app, so that it can reload itself with the new configuration" You could probably avoid the restart/reload by using skydns (https://github.com/skynetservices/skydns https://github.com/skynetservices/skydns), it is a local dns service built on top of values in etcd. You can configure your django app to access the other service containers by dns name and it will be translated to the correct ip all the time.

by jorlow 12y ago

HorizonXP 12y ago

Take Postgres for example. I use the official Docker image, and run it via a fleet .service file, having it store its data in a Docker data-only container, also launched from a fleet unit file. Finally, there's a third unit file that runs a slightly customized version of the docker-register image from CoreOS's polvi. This Dockerized app polls the Docker socket every 10s or so to get the IP and port information for the Docker container it's monitoring, in this case Postgres. It PUTs this value into etcd. Afterwards, when my Django containers start, they pull their configuration from etcd using python-etcd. It will get the IPs and ports for the services, including Postgres, from etcd, and configure itself using that. Finally, it will keep these values in a dictionary. If these values ever differ from what's currently in etcd, then my Django app will send a signal to cause uwsgi to gracefully restart the Django app, so that it can reload itself with the new configuration. For other Dockerized apps like nginx, I use kelseyhightower's confd-watch to monitor the etcd keys I'm interested in. confd-watch is great because it will use golang's text/template package to allow me to generate configuration files, like nginx.conf, with the values from etcd. So as these values get updated, so will nginx as confd-watch will force a graceful reload. Hope that helps.

leonidlm 12y ago
"If these values ever differ from what's currently in etcd, then my Django app will send a signal to cause uwsgi to gracefully restart the Django app, so that it can reload itself with the new configuration" You could probably avoid the restart/reload by using skydns (https://github.com/skynetservices/skydns https://github.com/skynetservices/skydns), it is a local dns service built on top of values in etcd. You can configure your django app to access the other service containers by dns name and it will be translated to the correct ip all the time.