Traffic Monitor

Introduction

Traffic Monitor is an HTTP service application that monitors caches, provides health state information to Traffic Router, and collects statistics for use in tools such as Traffic Portal and Traffic Stats. The health state provided by Traffic Monitor is used by Traffic Router to control which caches are available on the CDN.

Software Requirements

To work on Traffic Monitor you need a Unix-like (MacOS and Linux are most commonly used) environment that has a working install of Go

Project Tree Overview

traffic_monitor/ - base directory for Traffic Monitor.

  • cache/ - Handler for processing cache results.

  • config/ - Application configuration; in-memory objects from traffic_monitor.cfg.

  • crconfig/ - struct for deserlializing the CRConfig from JSON.

  • deliveryservice/ - aggregates Delivery Service data from cache results.

  • deliveryservicedata/ - Delivery Service structs. This exists separate from deliveryservice to avoid circular dependencies.

  • enum/ - enumerations and name alias types.

  • health/ - functions for calculating cache health, and creating health event objects.

  • manager/ - manager goroutines (microthreads).

    • health.go - Health request manager. Processes health results, from the health poller -> fetcher -> manager. The health poll is the “heartbeat” containing a small amount of stats, primarily to determine whether a cache is reachable as quickly as possible. Data is aggregated and inserted into shared threadsafe objects.
    • manager.go - Contains Start function to start all pollers, handlers, and managers.
    • monitorconfig.go - Monitor config manager. Gets data from the monitor config poller, which polls Traffic Ops for changes to which caches are monitored and how.
    • opsconfig.go - Ops config manager. Gets data from the ops config poller, which polls Traffic Ops for changes to monitoring settings.
    • peer.go - Peer manager. Gets data from the peer poller -> fetcher -> handler and aggregates it into the shared threadsafe objects.
    • stat.go - Stat request manager. Processes stat results, from the stat poller -> fetcher -> manager. The stat poll is the large statistics poll, containing all stats (such as HTTP codes, transactions, delivery service statistics, and more). Data is aggregated and inserted into shared threadsafe objects.
    • statecombiner.go - Manager for combining local and peer states, into a single combined states threadsafe object, for serving the CrStates endpoint.
  • datareq/ - HTTP routing, which has threadsafe health and stat objects populated by stat and health managers.

  • peer/ - Manager for getting and populating peer data from other Traffic Monitors

  • srvhttp/ - HTTP service. Given a map of endpoint functions, which are lambda closures containing aggregated data objects.

  • static/ - Web interface files (markup, styling and scripting)

  • threadsafe/ - Threadsafe objects for storing aggregated data needed by multiple goroutines (typically the aggregator and HTTP server)

  • trafficopsdata/ - Struct for fetching and storing Traffic Ops data needed from the CRConfig. This is primarily mappings, such as delivery service servers, and server types.

  • trafficopswrapper/ - Threadsafe wrapper around the Traffic Ops client. The client used to not be threadsafe, however, it mostly (possibly entirely) is now. But, the wrapper also serves to overwrite the Traffic Ops monitoring.json values, which are live, with snapshotted CRConfig values.

Architecture

At the highest level, Traffic Monitor polls caches, aggregates their data and availability, and serves it at HTTP endpoints in JSON format.

In the code, the data flows through microthread (goroutine) pipelines. All stages of the pipeline are independently running microthreads [1] . The pipelines are:

stat poll
Polls caches for all statistics data. This should be a slower poll, which gets a lot of data.
health poll
Polls caches for a tiny amount of data, typically system information. This poll is designed to be a heartbeat, determining quickly whether the cache is reachable. Since it’s a small amount of data, it should poll more frequently.
peer poll
Polls Traffic Monitor peers for their availability data, and aggregates it with its own availability results and that of all other peers.
monitor config
Polls Traffic Ops for the list of Traffic Monitors and their info.
ops config

Polls for changes to the ops config file traffic_ops.cfg, and sends updates to other pollers when the config file has changed.

  • The ops config manager also updates the shared Traffic Ops client, since it’s the actor which becomes notified of config changes requiring a new client.
  • The ops config manager also manages, creates, and recreates the HTTP server, since ops config changes necessitate restarting the HTTP server.

All microthreads in the pipeline are started by manager/manager.go:Start().

../_images/Pipeline.svg

Pipeline Overview

[1]Technically, some stages which are one-to-one simply call the next stage as a function. For example, the Fetcher calls the Handler as a function in the same microthread. But this isn’t architecturally significant.

Stat Pipeline

../_images/Stat_Pipeline.svg

The Stats Pipeline

poller
common/poller/poller.go:HttpPoller.Poll(). Listens for configuration changes (from the Ops Configuration Manager), and starts its own, internal microthreads - one for each cache to poll. These internal microthreads call the Fetcher at each cache’s poll interval.
fetcher
common/fetcher/fetcher.go:HttpFetcher.Fetch(). Fetches the given URL, and passes the returned data to the Handler, along with any errors.
handler
traffic_monitor/cache/cache.go:Handler.Handle(). Takes the given result and does all data computation possible with the single result. Currently, this computation primarily involves processing the de-normalized Apache Trafficserver (ATS) data into Go structs, and processing System data into ‘OutBytes’, ‘Kbps’, etc. Precomputed data is then passed to its result channel, which is picked up by the Manager.
manager
traffic_monitor/manager/stat.go:StartStatHistoryManager(). Takes preprocessed results, and aggregates them. Aggregated results are then placed in shared data structures. The major data aggregated are delivery service statistics, and cache availability data. See Aggregated Stat Data and Aggregated Availability Data.

Health Pipeline

../_images/Health_Pipeline.svg

The Health Pipeline

poller
common/poller/poller.go:HttpPoller.Poll(). Same poller type as the Stat Poller pipeline, with a different handler object.
fetcher
common/fetcher/fetcher.go:HttpFetcher.Fetch(). Same fetcher type as the Stat Poller pipeline, with a different handler object.
handler
traffic_monitor/cache/cache.go:Handler.Handle(). Same handler type as the Stat Poller pipeline, but constructed with a flag to not pre-compute anything. The health endpoint is of the same form as the stat endpoint, but doesn’t return all stat data. So, it doesn’t pre-compute like the Stat Handler, but only processes the system data, and passes the processed result to its result channel, which is picked up by the Manager.
manager
traffic_monitor/manager/health.go:StartHealthResultManager(). Takes preprocessed results, and aggregates them. For the Health pipeline, only health availability data is aggregated. Aggregated results are then placed in shared data structures (lastHealthDurationsThreadsafe, lastHealthEndTimes, etc). See Aggregated Availability Data.

Peer Pipeline

../_images/Peer_Pipeline.svg

The Peers Pipeline

poller
common/poller/poller.go:HttpPoller.Poll(). Same poller type as the Stat and Health Poller pipelines, with a different handler object. Its configuration changes come from the Monitor Configuration Manager, and it starts an internal microthread for each peer to poll.
fetcher
common/fetcher/fetcher.go:HttpFetcher.Fetch(). Same fetcher type as the Stat and Health Poller pipeline, with a different handler object.
handler
traffic_monitor/cache/peer.go:Handler.Handle(). Decodes the JSON result into an object, and without further processing passes to its result channel, which is picked up by the Manager.
manager
traffic_monitor/manager/peer.go:StartPeerManager(). Takes JSON peer Traffic Monitor results, and aggregates them. The availability of the Peer Traffic Monitor itself, as well as all cache availability from the given peer result, is stored in the shared peerStates object. Results are then aggregated via a call to the combineState() lambda, which signals the State Combiner microthread (which stores the combined availability in the shared object combinedStates; See State Combiner).

Monitor Config Pipeline

../_images/Monitor_Pipeline.svg

The Monitor Configuration Pipeline

poller
common/poller/poller.go:MonitorConfigPoller.Poll(). The Monitor Configuration poller, on its interval, polls Traffic Ops for the Monitor configuration, and writes the polled value to its result channel, which is read by the Manager.
manager
traffic_monitor/manager/monitorconfig.go:StartMonitorConfigManager(). Listens for results from the poller, and processes them. Cache changes are written to channels read by the Health, Stat, and Peer pollers. In the Shared Data objects, this also sets the list of new Delivery Services and removes ones which no longer exist, and sets the list of peer Traffic Monitors.

Ops Config Pipeline

../_images/Ops-Config_Pipeline.svg

The Ops Configuration Pipeline

poller
common/poller/poller.go:FilePoller.Poll(). Polls for changes to the Traffic Ops configuration file traffic_ops.cfg, and writes the changed configuration to its result channel, which is read by the Handler.
handler
common/handler/handler.go:OpsConfigFileHandler.Listen(). Takes the given raw configuration, un-marshals the JSON into an object, and writes the object to its channel, which is read by the Manager, along with any error.
manager
traffic_monitor/manager/monitorconfig.go:StartMonitorConfigManager(). Listens for new configurations, and processes them. When a new configuration is received, a new HTTP dispatch map is created via traffic_monitor/datareq/datareq.go:MakeDispatchMap(), and the HTTP server is restarted with the new dispatch map. The Traffic Ops client is also recreated, and stored in its shared data object. The Ops Configuration change subscribers and Traffic Ops Client change subscribers (the Monitor Configuration poller) are also passed the new ops configuration and new Traffic Ops client.

Events

The events shared data object is passed to each pipeline microthread which needs to signal events. Most of them do. Events are then logged, and visible in the UI as well as an HTTP JSON endpoint. Most events are caches becoming available or unavailable, but include other things such as peer availability changes.

State Combiner

The State Combiner is a microthread started in traffic_monitor/manager/manager.go:Start() via traffic_monitor/manager/statecombiner.go:StartStateCombiner(), which listens for signals to combine states. It should be signaled by any pipeline which updates the local or peer availability shared data objects, localStates and peerStates. It holds the thread-safe shared data objects for local states and peer states, so no data is passed or returned, only a signal. When a signal is received, it combines the local and peer states optimistically. That is, if a cache is marked available locally or by any peer, that cache is marked available in the combined states. There exists a variable to combine pessimistically, which may be set at compile time (it’s unusual for a CDN to operate well with pessimistic cache availability). Combined data is stored in the thread-safe shared data object combinedStates.

Aggregated Stat Data

The Stat pipeline Manager is responsible for aggregating stats from all caches, into delivery services statistics. This is done via a call to traffic_monitor/deliveryservice/stat.go:CreateStats().

Aggregated Availability Data

Both the Stat and Health pipelines aggregate availability data received from caches. This is done via a call to traffic_monitor/deliveryservice/health.go:CalcAvailability() followed by a call to combineState(). The CalcAvailability function calculates the availability of each cache from the result of polling it, that is, local availability. The combineState() function is a lambda passed to the Manager, which signals the State Combiner microthread, which will combine the local and peer Traffic Monitor availability data, and insert it into the shared data combinedStates object.

HTTP Data Requests

Data is provided to HTTP requests via the thread-safe shared data objects (see Shared Data). These objects are closed in lambdas created via traffic_monitor/datareq/datareq.go:MakeDispatchMap(). This is called by the Ops Configuration Manager when it recreates the HTTP server. Each HTTP endpoint is mapped to a function which closes around the shared data objects it needs, and takes the request data it needs (such as query parameters). Each endpoint function resides in its own file in traffic_monitor/datareq/. Because each Go HTTP routing function must be a http.HandlerFunc, wrapper functions take the endpoint functions and return http.HandlerFunc functions which call them, and which are stored in the dispatch map, to be registered with the HTTP server.

Shared Data

Processed and aggregated data must be shared between the end of the stat and health processing pipelines, and HTTP requests. The CSP paradigm of idiomatic Go does not work efficiently with storing and sharing state. While not idiomatic Go, shared mutexed data structures are faster and simpler than CSP manager microthreads for each data object. Traffic Monitor has many thread-safe shared data types and objects. All shared data objects can be seen in manager/manager.go:Start(), where they are created and passed to the various pipeline stage microthreads that need them. Their respective types all include the word Threadsafe, and can be found in traffic_monitor/threadsafe/ as well as, for dependency reasons, various appropriate directories. Currently, all thread-safe shared data types use mutexes. In the future, these could be changed to lock-free or wait-free structures, if the performance needs outweighed the readability and correctness costs. They could also easily be changed to internally be manager microthreads and channels, if being idiomatic were deemed more important than readability or performance.

Formatting Conventions

Go code should be formatted with gofmt. See also CONTRIBUTING.md.

Installing The Developer Environment

To install the Traffic Monitor Developer environment:

  1. Install Go version 1.7 or greater
  2. Clone the Traffic Control repository using git, into $GOPATH/src/github.com/apache/trafficcontrol
  3. Change directories into $GOPATH/src/github.com/apache/trafficcontrol/traffic_monitor
  4. Run ./build.sh

Test Cases

Tests can be executed by running go test ./... at the root of the traffic_monitor project.