Update README to contain blurb on Redis Cluster

README.md

@@ -142,6 +142,21 @@ func (c *Context) Export(job *work.Job) error {
 }
 ```
 
+## Redis Cluster
+If you're attempting to use gocraft/work on a `Redis Cluster` deployment, then you may encounter a `CROSSSLOT Keys in request don't hash to the same slot` error during the execution of the various lua scripts used to manage job data (see [Issue 93](https://github.com/gocraft/work/issues/93#issuecomment-401134340)). The current workaround is to force the keys for an entire `namespace` for a given worker pool on a single node in the cluster using [Redis Hash Tags](https://redis.io/topics/cluster-spec#keys-hash-tags). Using the example above:
+
+```go
+func main() {
+	// Make a new pool. Arguments:
+	// Context{} is a struct that will be the context for the request.
+	// 10 is the max concurrency
+	// "my_app_namespace" is the Redis namespace and the {} chars forces all of the keys onto a single node
+	// redisPool is a Redis pool
+	pool := work.NewWorkerPool(Context{}, 10, "{my_app_namespace}", redisPool)
+```
+
+*Note* this is not an issue for Redis Sentinel deployments.
+
 ## Special Features
 
 ### Contexts
@@ -206,6 +221,16 @@ pool.PeriodicallyEnqueue("0 0 * * * *", "calculate_caches") // This will enqueue
 pool.Job("calculate_caches", (*Context).CalculateCaches) // Still need to register a handler for this job separately
 ```
 
+## Job concurrency
+
+You can control job concurrency using `JobOptions{MaxConcurrency: <num>}`. Unlike the WorkerPool concurrency, this controls the limit on the number jobs of that type that can be active at one time by within a single redis instance. This works by putting a precondition on enqueuing function, meaning a new job will not be scheduled if we are at or over a job's `MaxConcurrency` limit. A redis key (see `redis.go::redisKeyJobsLock`) is used as a counting semaphore in order to track job concurrency per job type. The default value is `0`, which means "no limit on job concurrency".
+
+**Note:** if you want to run jobs "single threaded" then you can set the `MaxConcurrency` accordingly:
+```go
+      worker_pool.JobWithOptions(jobName, JobOptions{MaxConcurrency: 1}, (*Context).WorkFxn)
+```
+
+
 ## Run the Web UI
 
 The web UI provides a view to view the state of your gocraft/work cluster, inspect queued jobs, and retry or delete dead jobs.
@@ -291,23 +316,11 @@ You'll see a view that looks like this:
 * Each worker pool will wake up every 2 minutes, and if jobs haven't been scheduled yet, it will schedule all the jobs that would be executed in the next five minutes.
 * Each periodic job that runs at a given time has a predictable byte pattern. Since jobs are scheduled on the scheduled job queue (a Redis z-set), if the same job is scheduled twice for a given time, it can only exist in the z-set once.
 
-## Paused jobs
+### Paused jobs
 
 * You can pause jobs from being processed from a specific queue by setting a "paused" redis key (see `redisKeyJobsPaused`)
 * Conversely, jobs in the queue will resume being processed once the paused redis key is removed
 
-## Job concurrency
-
-* You can control job concurrency using `JobOptions{MaxConcurrency: <num>}`.
-* Unlike the WorkerPool concurrency, this controls the limit on the number jobs of that type that can be active at one time by within a single redis instance
-* This works by putting a precondition on enqueuing function, meaning a new job will not be scheduled if we are at or over a job's `MaxConcurrency` limit
-* A redis key (see `redisKeyJobsLock`) is used as a counting semaphore in order to track job concurrency per job type
-* The default value is `0`, which means "no limit on job concurrency"
-* **Note:** if you want to run jobs "single threaded" then you can set the `MaxConcurrency` accordingly:
-```go
-      worker_pool.JobWithOptions(jobName, JobOptions{MaxConcurrency: 1}, (*Context).WorkFxn)
-```
-
 ### Terminology reference
 * "worker pool" - a pool of workers
 * "worker" - an individual worker in a single goroutine. Gets a job from redis, does job, gets next job...