The Things Network HTTP Integration Part12

Removing the DIY cache

For the Proof of Concept(PoC) I had written a simple cache using a ConcurrentDictionary to store Azure IoT Hub connections to reduce the number of calls to the Device Provisioning Service(DPS).

Device Provisioning Service calls in stress test

For a PoC the DIY cache was ok but I wanted to replace it with something more robust like the .Net ObjectCache which is in the System.Runtime.Caching namespace.

I started by replacing the ConcurrentDictionary declaration

static readonly ConcurrentDictionary<string, DeviceClient> DeviceClients = new ConcurrentDictionary<string, DeviceClient>();

With an ObjectCache declaration.

static readonly ObjectCache DeviceClients = MemoryCache.Default;

Then, where there were compiler errors I updated the method call.

// See if the device has already been provisioned or is being provisioned on another thread.
if (DeviceClients.Add(registrationId, deviceContext, cacheItemPolicy))
   log.LogInformation("RegID:{registrationId} Device provisioning start", registrationId);

One difference I found was that ObjectCache throws an exception if the value is null. I was using a null value to indicate that the Device Provisioning Service(DPS) process had been initiated on another thread and was underway.

I have been planning to add support for downlink messages so I added a new class to store the uplink (Azure IoT Hub DeviceClient) and downlink ( downlink_url in the uplink message) details.

 public class DeviceContext
      public DeviceClient Uplink { get; set; }
      public Uri Downlink { get; set; }

For the first version the only functionality I’m using is sliding expiration which is set to one day

CacheItemPolicy cacheItemPolicy = new CacheItemPolicy()
   SlidingExpiration = new TimeSpan(1, 0, 0, 0),

DeviceContext deviceContext = new DeviceContext()
   Uplink = null,
   Downlink = new Uri(payload.DownlinkUrl)

I didn’t have to make many changes and I’ll double check my implementation in the next round of stress and soak testing.

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