How to diagnose performance problems
In a web applications, a performance problem is generally experienced in two ways: as a lack of speed in responding to client requests, or unexpected errors from the site.
Errors caused by performance problems are typically (but not limited to) Application not responding or Gateway timeout errors.
Almost without exception, performance problems with sites running on Divio are caused by problems originating within the applications themselves, and are not related to the performance of our servers, network or infrastructure.
We monitor all of our services around the clock. Any components whose performance or operation falls outside safe parameters will immediately raise an alert, and will be checked by our SRE team. Many of those components will automatically be recycled or restarted by our monitoring systems.
In the case of any on-going issue affecting Divio infrastructure, we will post a notice on https://status.divio.com.
Action without information is useless
It is almost impossible to address any performance problem without adequately understanding its causes. Any action attempting to resolve the problem without a good understanding of its nature is simply a shot in the dark with little chance of success.
It is tempting to consider upgrading a subscription (i.e. increasing resources such as RAM and CPU) as an immediate first resort, but - as much as Divio will be pleased to see you spending more money on our services - this should be resisted.
Use Application Performance Monitoring
The single most useful thing at your disposal when trying to identify the cause of a performance issue is an Application Performance Monitoring tool. There is nothing that provides an equally valuable insight into what an application is actually doing - how long it spends dealing with a particular request, or kind of request, and the code pathways a request invokes.
It can quickly become apparent whether a slow response is being held up in straightforward computational processing, database or file interaction, template rendering or other process. (For example, if it turns out that a certain request triggers a very expensive database query, then there is no amount of other resources that can be thrown at the application to improve performance: the only solution is to optimise the query, or ensure that its results can be cached as much as possible. If the query takes so long to process that the request times out before the database can return a result, then only rewriting the query will be solution.)
Well-established APM solutions suitable for web applications include New Relic and Elastic APM. Most well-known services offer integration packages for multiple different languages.
For any mission-critical application or serious effort to identify performance shortfalls, we strongly recommend using APM. A little investment in APM can save you great expenditure in time, effort and money.
Use other tools
All other tools are second-best to APM, and none of them can take its place. However, there are numerous other options that help provide some level of insight all the same.
Metrics
All Divio applications include metrics. As well as showing whether an application is consuming excessive resources, metrics can also show when changes in consumption occurred (for example, following deployment of new code).
In most cases, users will find that their under-performing application is not in fact running out of RAM or CPU - a clear indication that adding resources by upgrading a subscription will not be a solution.
You may find that RAM and/or CPU consumption are reaching exceeding expected values. These can both in themselves cause degraded performance. In that case, it is possible that the application should be allocated more resources, but it is equally likely that finding ways to reduce its resource consumption will be a better solution (a memory leak for example will not be solved by adding more RAM).
Runtime logs
All Divio applications provide runtime logs, accessible from the Environments view as well as from the terminal (e.g.
divio app logs --tail live). Logs show for example how long requests take to serve and reporting internal
errors, but this provides only a basic insight into behaviour. However, as with metrics, referring to runtime logs can
help eliminate suspects from your enquiries quite quickly.
Debug tools
Debug tools (such as Django Debug Toolbar), though not fully-fledged APM products, can also provide a degree of insight into an application's performance, especially when investigating the behaviour of an application when dealing with a particular request, and can be easier to set up and use. They are especially useful in development environments.