LEdoian
/
recodex-wiki
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Copy broker content into new doc

Browse Source
master
Martin Polanka 8 years ago
parent 46f6b38bb7
commit abe471833b
2 changed files with 68 additions and 74 deletions
Show Stats Download Patch File Download Diff File

72
Broker.md
Unescape Escape View File

@ -1,72 +0,0 @@
# Broker
The broker is a central part of the ReCodEx backend that directs almost all
communication. It was designed to properly maintain heavy load of messages
by making only small actions in main communication thread and asynchronous
execution of other actions.
## Description
The broker's responsibilites are:
- allowing workers to register themselves and keep track of their capabilities
- tracking status of each worker and handle cases when they crash
- accepting assignment evaluation requests from the frontend and forwarding them
to workers
- receiving job status information from workers and forward it to the frontend
either via monitor or REST API
- notifying the frontend of errors in the backend
## Architecture
The broker uses our ZeroMQ _reactor_ to bind events on sockets to handler classes.
There are currently two handlers -- one that handles the main functionality and
another one that sends status reports to the REST API asynchronously so that the
broker does not have to wait for HTTP requests which can take a lot of time,
especially when some kind of error happens on the server.
### Worker registry
The `worker_registry` class is used to store information about workers, their
status and the jobs in their queue. It can look up a worker using the headers
received with a request (a worker is considered suitable if and only if it
satisfies all the headers). The headers are arbitrary key-value pairs, which
are checked for equality by the broker. However, some headers require special
handling, namely `threads`, for which we check if the value in the request is
lesser than or equal to the value advertised by the worker, and `hwgroup`, for
which we support requesting one of multiple hardware groups by listing multiple
names separated with a `|` symbol (e.g. `group_1|group_2|group_3`.
The registry also implements a basic load balancing algorithm -- the
workers are contained in a queue and whenever one of them receives a job, it is
moved to its end, which makes it less likely to receive another job soon.
When a worker is assigned a job, it will not be assigned another one until we
receive a `done` message from it.
### Error handling
**Job failure** -- we recognize two ways a job can fail -- an internally and
externally. An internal failure is the worker's fault -- for example when it
cannot download a file needed for the evaluation for some reason. An external
error is for example when the job configuration is malformed. Note that we do not
consider a student entering an incorrect solution a job failure.
Jobs that failed internally are reassigned until a limit on the amount of
reassingments (configurable with the `max_request_failures` option) is reached.
External failures are reported to the frontend immediately.
**Worker failure** -- when a worker crash is detected, we attempt to reassign its
current job and also all the jobs from its queue. Because the current job might
be the reason of the crash, its reassignment is also counted towards the
`max_request_failures` limit (the counter is shared). If there is no worker that
could process a job (i.e. it cannot be reassigned), the job is reported as
failed to the frontend via REST API.
**Broker failure** -- when the broker itself crashed and is restarted, workers
will reconnect automatically. However, all jobs in their queues are lost. If a
worker manages to finish a job and notifies the "new" broker, the report is
forwarded to the frontend. The same goes for external failures. Jobs that fail
internally cannot be reassigned, because the "new" broker does not know their
headers -- they are reported as failed immediately.

70
Rewritten-docs.md
Unescape Escape View File

@ -2656,10 +2656,76 @@ used.
## Broker
@todo: gets stuff done, single point of failure and center point of ReCodEx universe
The broker is a central part of the ReCodEx backend that directs almost all
communication. It was designed to properly maintain heavy load of messages by
making only small actions in main communication thread and asynchronous
execution of other actions.
The responsibilites of broker are:
- allowing workers to register themselves and keep track of their capabilities
- tracking status of each worker and handle cases when they crash
- accepting assignment evaluation requests from the frontend and forwarding them
to workers
- receiving job status information from workers and forward it to the frontend
either via monitor or REST API
- notifying the frontend of errors in the backend
### Architecture
The broker uses our ZeroMQ _reactor_ to bind events on sockets to handler
classes. There are currently two handlers -- one that handles the main
functionality and another one that sends status reports to the REST API
asynchronously so that the broker does not have to wait for HTTP requests which
can take a lot of time, especially when some kind of error happens on the
server.
#### Worker registry
The `worker_registry` class is used to store information about workers, their
status and the jobs in their queue. It can look up a worker using the headers
received with a request (a worker is considered suitable if and only if it
satisfies all the headers). The headers are arbitrary key-value pairs, which are
checked for equality by the broker. However, some headers require special
handling, namely `threads`, for which we check if the value in the request is
lesser than or equal to the value advertised by the worker, and `hwgroup`, for
which we support requesting one of multiple hardware groups by listing multiple
names separated with a `|` symbol (e.g. `group_1|group_2|group_3`.
The registry also implements a basic load balancing algorithm -- the workers are
contained in a queue and whenever one of them receives a job, it is moved to its
end, which makes it less likely to receive another job soon.
When a worker is assigned a job, it will not be assigned another one until we
receive a `done` message from it.
#### Error handling
**Job failure** -- we recognize two ways a job can fail -- an internally and
externally. An internal failure is the fault of worker -- for example when it
cannot download a file needed for the evaluation for some reason. An external
error is for example when the job configuration is malformed. Note that we do
not consider a student entering an incorrect solution a job failure.
Jobs that failed internally are reassigned until a limit on the amount of
reassingments (configurable with the `max_request_failures` option) is reached.
External failures are reported to the frontend immediately.
**Worker failure** -- when a worker crash is detected, we attempt to reassign
its current job and also all the jobs from its queue. Because the current job
might be the reason of the crash, its reassignment is also counted towards the
`max_request_failures` limit (the counter is shared). If there is no worker
that could process a job (i.e. it cannot be reassigned), the job is reported as
failed to the frontend via REST API.
**Broker failure** -- when the broker itself crashed and is restarted, workers
will reconnect automatically. However, all jobs in their queues are lost. If a
worker manages to finish a job and notifies the "new" broker, the report is
forwarded to the frontend. The same goes for external failures. Jobs that fail
internally cannot be reassigned, because the "new" broker does not know their
headers -- they are reported as failed immediately.
@todo: what to mention:
- job scheduling, worker queues
- API notification using curl, authentication using HTTP Basic Auth
- asynchronous resending progress messages

Write Preview
Loading…
Cancel
Save