3 /* A WorkQueue is an asynchronous thread-safe queue manager. It
4 provides a channel from which items can be read off the queue, and
5 permits replacing the contents of the queue at any time.
7 The overall work flow for a WorkQueue is as follows:
9 1. A WorkQueue is created with NewWorkQueue(). This
10 function instantiates a new WorkQueue and starts a manager
11 goroutine. The manager listens on an input channel
12 (manager.newlist) and an output channel (manager.NextItem).
14 2. The manager first waits for a new list of requests on the
15 newlist channel. When another goroutine calls
16 manager.ReplaceQueue(lst), it sends lst over the newlist
17 channel to the manager. The manager goroutine now has
18 ownership of the list.
20 3. Once the manager has this initial list, it listens on both the
21 input and output channels for one of the following to happen:
23 a. A worker attempts to read an item from the NextItem
24 channel. The manager sends the next item from the list
25 over this channel to the worker, and loops.
27 b. New data is sent to the manager on the newlist channel.
28 This happens when another goroutine calls
29 manager.ReplaceItem() with a new list. The manager
30 discards the current list, replaces it with the new one,
31 and begins looping again.
33 c. The input channel is closed. The manager closes its
34 output channel (signalling any workers to quit) and
37 Tasks currently handled by WorkQueue:
43 // Any kind of user-defined type can be used with the
45 type FrobRequest struct {
50 froblist := NewWorkQueue()
52 // Start a concurrent worker to read items from the NextItem
53 // channel until it is closed, deleting each one.
54 go func(list WorkQueue) {
55 for i := range list.NextItem {
56 req := i.(FrobRequest)
61 // Set up a HTTP handler for PUT /frob
62 router.HandleFunc(`/frob`,
63 func(w http.ResponseWriter, req *http.Request) {
64 // Parse the request body into a list.List
65 // of FrobRequests, and give this list to the
67 newfrobs := parseBody(req.Body)
68 froblist.ReplaceQueue(newfrobs)
71 Methods available on a WorkQueue:
74 Replaces the current item list with a new one. The list
75 manager discards any unprocessed items on the existing
76 list and replaces it with the new one. If the worker is
77 processing a list item when ReplaceQueue is called, it
78 finishes processing before receiving items from the new
81 Shuts down the manager goroutine. When Close is called,
82 the manager closes the NextItem channel.
85 import "container/list"
87 type WorkQueue struct {
88 countInProgress chan int
89 countOutstanding chan int
91 newlist chan *list.List
92 // Workers get work items by reading from this channel.
93 NextItem <-chan interface{}
94 // Each worker must send struct{}{} to ReportDone exactly once
95 // for each work item received from NextItem, when it stops
96 // working on that item (regardless of whether the work was
98 ReportDone chan<- struct{}
101 // NewWorkQueue returns a new empty WorkQueue.
103 func NewWorkQueue() *WorkQueue {
104 nextItem := make(chan interface{})
105 reportDone := make(chan struct{})
106 newList := make(chan *list.List)
108 countQueued: make(chan int),
109 countInProgress: make(chan int),
110 countOutstanding: make(chan int),
113 ReportDone: reportDone,
116 // Read new work lists from the newlist channel.
117 // Reply to "length" and "get next item" queries by
118 // sending to the countQueued and nextItem channels
119 // respectively. Return when the newlist channel
125 // When we're done, close the output channel; workers will
126 // shut down next time they ask for new work.
127 defer close(nextItem)
128 defer close(b.countInProgress)
129 defer close(b.countOutstanding)
130 defer close(b.countQueued)
132 var nextChan chan interface{}
133 var nextVal interface{}
134 for newList != nil || countInProgress > 0 {
136 case p, ok := <-newList:
138 // Closed, stop receiving
151 nextVal = todo.Front().Value
153 case nextChan <- nextVal:
155 todo.Remove(todo.Front())
161 nextVal = todo.Front().Value
165 case b.countInProgress <- countInProgress:
166 case b.countOutstanding <- todo.Len() + countInProgress:
167 case b.countQueued <- todo.Len():
174 // ReplaceQueue abandons any work items left in the existing queue,
175 // and starts giving workers items from the given list. After giving
176 // it to ReplaceQueue, the caller must not read or write the given
179 func (b *WorkQueue) ReplaceQueue(list *list.List) {
183 // Close shuts down the manager and terminates the goroutine, which
184 // abandons any pending requests, but allows any pull request already
185 // in progress to continue.
187 // After Close, CountX methods will return correct values, NextItem
188 // will be closed, and ReplaceQueue will panic.
190 func (b *WorkQueue) Close() {
194 // CountOutstanding returns the number of items in the queue or in
195 // progress. A return value of 0 guarantees all existing work (work
196 // that was sent to ReplaceQueue before CountOutstanding was called)
199 func (b *WorkQueue) CountOutstanding() int {
200 // If the channel is closed, we get zero, which is correct.
201 return <-b.countOutstanding
204 // CountQueued returns the number of items in the current queue.
206 func (b *WorkQueue) CountQueued() int {
207 return <-b.countQueued
210 // Len returns the number of items in progress.
212 func (b *WorkQueue) CountInProgress() int {
213 return <-b.countInProgress