public static Result <ModelOutput> Predict(RedisClusterSize currentClusterSize, ModelContext modelContext)
{
var shortestPaths = ComputeAllowedPaths(currentClusterSize, modelContext);
var eligibleClusterSizes = shortestPaths
.Select(kvp => (Size: kvp.Key, Node: kvp.Value))
// Find all plans that we can reach from the current one via scaling operations, and that we allow scaling to
.Where(entry => entry.Node.ShortestDistanceFromSource != double.PositiveInfinity && IsScalingAllowed(currentClusterSize, entry.Size, modelContext))
// Compute the cost of taking the given route
.Select(entry => (entry.Size, entry.Node, Cost: CostFunction(currentClusterSize, entry.Size, modelContext, shortestPaths)))
.ToList();
// Rank them by cost ascending
var costSorted = eligibleClusterSizes
.OrderBy(pair => pair.Cost)
.ToList();
if (costSorted.Count == 0)
{
return(new Result <ModelOutput>(errorMessage: "No cluster size available for scaling"));
}
return(new ModelOutput(
targetClusterSize: costSorted[0].Size,
modelContext: modelContext,
cost: costSorted[0].Cost,
scalePath: RedisScalingUtilities.ComputeShortestPath(shortestPaths, currentClusterSize, costSorted[0].Size)));
}
private Task <BoolResult> RequestScaleAsync(OperationContext context, RedisClusterSize targetClusterSize)
{
string extraMessage = $"CurrentClusterSize=[{ClusterSize}] TargetClusterSize=[{targetClusterSize}]";
return(context.PerformOperationAsync(Tracer, async() =>
{
if (ClusterSize.Equals(targetClusterSize))
{
return new BoolResult(errorMessage: $"No-op scale request attempted (`{ClusterSize}` -> `{targetClusterSize}`) on instance `{Name}`");
}
if (!RedisScalingUtilities.CanScale(ClusterSize, targetClusterSize))
{
return new BoolResult(errorMessage: $"Scale request `{ClusterSize}` -> `{targetClusterSize}` on instance `{Name}` is disallowed by Azure Cache for Redis");
}
if (!IsReadyToScale)
{
return new BoolResult(errorMessage: $"Redis instance `{Name}` is not ready to scale, current provisioning state is `{RedisCache.ProvisioningState}`");
}
var instance = RedisCache.Update();
if (!ClusterSize.Tier.Equals(targetClusterSize.Tier))
{
switch (targetClusterSize.Tier.Plan)
{
case RedisPlan.Basic:
instance = instance.WithBasicSku(targetClusterSize.Tier.Capacity);
break;
case RedisPlan.Standard:
instance = instance.WithStandardSku(targetClusterSize.Tier.Capacity);
break;
case RedisPlan.Premium:
instance = instance.WithPremiumSku(targetClusterSize.Tier.Capacity);
break;
}
}
if (ClusterSize.Shards != targetClusterSize.Shards)
{
instance = instance.WithShardCount(targetClusterSize.Shards);
}
await instance.ApplyAsync(context.Token);
return BoolResult.Success;
},
extraStartMessage: extraMessage,
extraEndMessage: _ => extraMessage,
pendingOperationTracingInterval: TimeSpan.FromMinutes(1)));
}
public static IReadOnlyDictionary <RedisClusterSize, RedisScalingUtilities.Node> ComputeAllowedPaths(RedisClusterSize currentClusterSize, ModelContext modelContext)
{
// We need to reach the target cluster size, but we can't do it in one shot because business rules won't
// let us, so we need to compute a path to get to it. This is probably the most complex part of the
// algorithm, there are several competing aspects we want to optimize for, in descending importance:
// - We want for memory to get to the target level ASAP
// - We want to keep the number of shards as stable as possible, given that changing them can cause build
// failures
// - We'd like to get there in the fewest amount of time possible
// - The route needs to be deterministic, so that if we are forced to stop and re-compute it we'll take
// the same route.
// - We'd like to minimize the cost of the route
// Multi-constraint optimization over graphs is NP-complete and algorithms are hard to come up with, so we
// do our best.
Func <RedisClusterSize, IEnumerable <RedisClusterSize> > neighbors =
currentClusterSize => currentClusterSize.ScaleEligibleSizes.Where(targetClusterSize =>
{
// Constrain paths to downscale at most one shard at the time. This only makes paths longer, so it
// is safe. The reason behind this is that the service doesn't really tolerate big reductions.
if (targetClusterSize.Shards < currentClusterSize.Shards)
{
return(targetClusterSize.Shards == currentClusterSize.Shards - 1);
}
return(true);
});
Func <RedisClusterSize, RedisClusterSize, double> weight =
(from, to) =>
{
// This factor is used to avoid transitioning to any kind of intermediate plan that may cause a
// production outage. If we don't have it, we may transition into a state in which we have less
// cluster memory available than we need. By adjusting the weight function, we guarantee that
// this only happens iff there is no better path; moreover, we will always choose the lesser of
// two evils if given no choice.
double clusterMemoryPenalization = 0;
var delta = to.ClusterMemorySizeMb - modelContext.MinimumAllowedClusterMemoryMb;
if (delta < 0)
{
// The amount of cluster memory is less than we need, so we penalize taking this path by
// adding the amount of memory that keeps us away from the target.
clusterMemoryPenalization = -delta;
}
// This needs to be at least one so we don't pick minimum paths that are arbitrarily long
return(1 + clusterMemoryPenalization);
};
return(RedisScalingUtilities.ComputeOneToAllShortestPath(vertices: RedisClusterSize.Instances, neighbors: neighbors, weight: weight, from: currentClusterSize));
}
/// <summary>
/// This function embodies the concept of "how much does it cost to switch from
/// <paramref name="current"/> to <paramref name="target"/>". At this point, we can assume that:
/// - The two input sizes are valid states to be in
/// - We can reach the target from current via some amount of autoscaling operations
/// Hence, we're just ranking amonst the many potential states.
/// </summary>
public static double CostFunction(RedisClusterSize current, RedisClusterSize target, ModelContext modelContext, IReadOnlyDictionary <RedisClusterSize, RedisScalingUtilities.Node> shortestPaths)
{
// Switching to the same size (i.e. no op) is free
if (current.Equals(target))
{
return(0);
}
var shortestPath = RedisScalingUtilities.ComputeShortestPath(shortestPaths, current, target);
Contract.Assert(shortestPath.Count > 0);
// Positive if we are spending more money, negative if we are saving
return((double)(target.MonthlyCostUsd - current.MonthlyCostUsd));
}
private async Task <BoolResult> RequestScaleAsync(RedisClusterSize targetClusterSize, CancellationToken cancellationToken = default)
{
if (ClusterSize.Equals(targetClusterSize))
{
return(new BoolResult(errorMessage: $"No-op scale request attempted (`{ClusterSize}` -> `{targetClusterSize}`) on instance `{Name}`"));
}
if (!RedisScalingUtilities.CanScale(ClusterSize, targetClusterSize))
{
return(new BoolResult(errorMessage: $"Scale request `{ClusterSize}` -> `{targetClusterSize}` on instance `{Name}` is disallowed by Azure Cache for Redis"));
}
if (!IsReadyToScale)
{
return(new BoolResult(errorMessage: $"Redis instance `{Name}` is not ready to scale, current provisioning state is `{RedisCache.ProvisioningState}`"));
}
var instance = RedisCache.Update();
if (!ClusterSize.Tier.Equals(targetClusterSize.Tier))
{
switch (targetClusterSize.Tier.Plan)
{
case RedisPlan.Basic:
instance = instance.WithBasicSku(targetClusterSize.Tier.Capacity);
break;
case RedisPlan.Standard:
instance = instance.WithStandardSku(targetClusterSize.Tier.Capacity);
break;
case RedisPlan.Premium:
instance = instance.WithPremiumSku(targetClusterSize.Tier.Capacity);
break;
}
}
if (ClusterSize.Shards != targetClusterSize.Shards)
{
instance = instance.WithShardCount(targetClusterSize.Shards);
}
await instance.ApplyAsync(cancellationToken);
return(BoolResult.Success);
}
private async Task <BoolResult> RequestScaleAsync(RedisClusterSize targetClusterSize, CancellationToken cancellationToken = default)
{
if (ClusterSize.Equals(targetClusterSize))
{
return(new BoolResult(errorMessage: $"No-op scale request attempted (`{ClusterSize}` -> `{targetClusterSize}`) on instance `{Name}`"));
}
if (!RedisScalingUtilities.CanScale(ClusterSize, targetClusterSize))
{
return(new BoolResult(errorMessage: $"Scale request `{ClusterSize}` -> `{targetClusterSize}` on instance `{Name}` is disallowed by Azure Cache for Redis"));
}
if (!IsReadyToScale)
{
return(new BoolResult(errorMessage: $"Redis instance `{Name}` is not ready to scale, current provisioning state is `{RedisCache.ProvisioningState}`"));
}
return(await SubmitScaleRequestAsync(targetClusterSize, cancellationToken));
}
private async Task <bool> AttemptToScaleAsync(RuleContext context, IRedisInstance redisInstance)
{
Contract.Requires(redisInstance.IsReadyToScale);
var operationContext = context.IntoOperationContext(_configuration.Logger);
// Fetch which cluster size we want, and start scaling operation if needed.
var currentClusterSize = redisInstance.ClusterSize;
var targetClusterSizeResult = await _redisAutoscalingAgent.EstimateBestClusterSizeAsync(operationContext, redisInstance);
if (!targetClusterSizeResult.Succeeded)
{
Emit(context, "Autoscale", Severity.Error, $"Failed to find best plan for instance `{redisInstance.Name}` in plan `{currentClusterSize}`. Result=[{targetClusterSizeResult}]");
return(false);
}
var modelOutput = targetClusterSizeResult.Value;
Contract.AssertNotNull(modelOutput);
var targetClusterSize = modelOutput.TargetClusterSize;
if (targetClusterSize.Equals(currentClusterSize) || modelOutput.ScalePath.Count == 0)
{
return(false);
}
if (RedisScalingUtilities.IsDownScale(currentClusterSize, targetClusterSize))
{
// Downscales are typically about saving money rather than system health, hence, it's deprioritized.
// Force downscales to happen during very comfortable business hours in PST, to ensure we're always
// available if things go wrong. We disregard holidays because it's a pain to handle them.
if (!TimeConstraints.BusinessHours.SatisfiedPST(_configuration.Clock.UtcNow))
{
Emit(context, "Autoscale", Severity.Info, $"Refused autoscale from `{currentClusterSize}` to `{targetClusterSize}` via scale path `{currentClusterSize} -> {string.Join(" -> ", modelOutput.ScalePath)}` for instance `{redisInstance.Name}` due to business hours constraints");
return(false);
}
// Downscales are performed in phases instead of all at once. If the model proposes an autoscale, we'll
// only take the first step of it in the current iteration, and force wait some amount of time until we
// allow this instance to be downscaled again. This gives some time to evaluate the effects of the last
// downscale (which typically takes time because migration's effects on instance memory and cpu load
// take some time to see).
//
// The intent of this measure is to avoid situations where our downscale causes heightened load in the
// remaining shards, forcing us to scale back to our original size after some time. This effect creates
// "autoscale loops" over time.
modelOutput.ScalePath = modelOutput.ScalePath.Take(1).ToList();
if (_lastAutoscaleTimeUtc.TryGetValue(redisInstance.Id, out var lastAutoscaleTimeUtc))
{
var now = _configuration.Clock.UtcNow;
if (now - lastAutoscaleTimeUtc < _configuration.MinimumWaitTimeBetweenDownscaleSteps)
{
return(false);
}
}
}
Emit(context, "Autoscale", Severity.Warning, $"Autoscaling from `{currentClusterSize}` ({currentClusterSize.MonthlyCostUsd} USD/mo) to `{targetClusterSize}` ({targetClusterSize.MonthlyCostUsd} USD/mo) via scale path `{currentClusterSize} -> {string.Join(" -> ", modelOutput.ScalePath)}` for instance `{redisInstance.Name}`. CostFunction=[{modelOutput.Cost}]");
var scaleResult = await redisInstance.ScaleAsync(operationContext, modelOutput.ScalePath);
_lastAutoscaleTimeUtc[redisInstance.Id] = _configuration.Clock.UtcNow;
if (!scaleResult)
{
Emit(context, "Autoscale", Severity.Error, $"Autoscale attempt from `{currentClusterSize}` to `{targetClusterSize}` for instance `{redisInstance.Name}` failed. Result=[{scaleResult}]");
scaleResult.ThrowIfFailure();
}
return(true);
}
public RedisClusterSize(RedisTier tier, int shards)
{
Contract.Requires(1 <= shards && shards <= 10, "Number of shards out of bounds, must be between 1 and 10");
Tier = tier;
Shards = shards;
_scaleEligibleSizes = new Lazy <IReadOnlyList <RedisClusterSize> >(() => Instances.Where(to => RedisScalingUtilities.CanScale(this, to)).ToList(), System.Threading.LazyThreadSafetyMode.PublicationOnly);
}
