Event when a KV value changes

[arpontes]

Hello,

I'm using consuldotnet for a long time, but when I need to check if a value changes, I need to do it on my own. I think would be nice if this lib could do a check every X minutes for a change in the value and then fire and event with the new value.

I could implement this funcionality and send an PR, if you approve such feature.

[mfkl]

Hi,

Not sure that's in scope with consuldotnet, but maybe. Could you share a snippet of the solution you are currently using please?

I'm using consuldotnet for a long time, but when I need to check if a value changes, I need to do it on my own.

Do you use consul watch for this or your own code?

If you suggest custom code for this, then users will probably ask next for a HTTP retry/error handling mechanisms, internet connection check, etc. I'm not sure this is in scope with this project, but let's see what others say.

Relevant:

• https://medium.com/@joatmon08/manage-monitor-consul-kv-changes-47e2a7a2d73d
• https://www.consul.io/commands/watch
• https://www.consul.io/docs/dynamic-app-config/watches

[lucascebertin]

Well, just trying to compare consul utilization across some languages and nodejs lib brings a watch functionality. https://github.com/silas/node-consul#watch

Snippet:

import consulFn from 'consul'

const consul = consulFn()
const watch = consul.watch({
  method: consul.kv.get,
  options: { key: 'test' },
  backoffFactor: 1000,
});

watch.on('change', function(data, res) {
  console.log('data:', data);
});

watch.on('error', function(err) {
  console.log('error:', err);
});

setTimeout(function() { watch.end(); }, 30 * 1000);

@arpontes, any chance to this being related? Could you share how you do that in dotnet today?

Thanks

[marcin-krystianc]

Consul supports blocking HTTP queries, so it is entirely possible to watch for value changes in a very efficient fashion. When the watched value changes, the HTTP call returns instantaneously - see a simplified example implementation.:

var lastIndex = 0ul;
do
{
  var res = await _client.KV.Get("foo", new QueryOptions { WaitIndex = lastIndex });
  // TODO - value of "foo" has changed
} while(true);

As mfkl has already mentioned, the tricky bit is exposing in the API a configurable error handling and retrying policy.

[chester89]

I'd like watch functionality for service instances. Will the trick similar to KV work? e.g. var result = await consulClient.Health.Service("serviceName", "tagName", true, new QueryOptions { WaitIndex = 0ul }, cancellationToken); //further processing

[marcin-krystianc]

I'd like watch functionality for service instances. Will the trick similar to KV work? e.g. var result = await consulClient.Health.Service("serviceName", "tagName", true, new QueryOptions { WaitIndex = 0ul }, cancellationToken); //further processing

Yes, I think it will work. Mind, that WaitIndex need to be set to the value from the previous query result.