Merge pull request #100 from prometheus/improve-config-docs

Improve Configuration / SD documentation.

content/docs/operating/configuration.md

@@ -15,8 +15,8 @@ load](/docs/querying/rules/#configuring-rules).
 To view all available command-line flags, run `prometheus -h`.
 
 Prometheus can reload its configuration at runtime. If the new configuration
-is not well-formatted the changes will not be applied.
-A configuration reload is triggered by sending `SIGHUP` to the Prometheus process.
+is not well-formed, the changes will not be applied.
+A configuration reload is triggered by sending a `SIGHUP` to the Prometheus process.
 This will also reload any configured rule files.
 
 
@@ -40,7 +40,7 @@ The other placeholders are specified separately.
 
 A valid example file can be found [here](https://github.com/prometheus/prometheus/blob/master/config/testdata/conf.good.yml).
 
-The global configuration specifies parameters valid in all other configuration
+The global configuration specifies parameters that are valid in all other configuration
 contexts. They also serve as defaults for other configuration sections.
 
 
@@ -71,14 +71,15 @@ scrape_configs:
 
 ### Scrape configurations `<scrape_config>`
 
-The scrape config specifies a set of targets, which might dynamically change, and
-parameters describing how to scrape them.
-In the general case one scrape configuration specifies a single job. In advanced
-configurations this might change.
+A `scrape_config` section specifies a set of targets and parameters describing how
+to scrape them. In the general case, one scrape configuration specifies a single
+job. In advanced configurations, this may change.
 
-Static targets can be configured via the `target_groups` parameter. The other
-configs allow dynamic target discovery. Additionally, the `relabel_configs` allow
-advanced modifications to any target belonging to the scrape config.
+Targets may be statically configured via the `target_groups` parameter or
+dynamically discovered using one of the supported service-discovery mechanisms.
+
+Additionally, `relabel_configs` allow advanced modifications to any
+target and its labels before scraping.
 
 ```
 # The job name assigned to scraped metrics by default.
@@ -129,8 +130,8 @@ regex `[a-zA-Z_][a-zA-Z0-9_-]`.
 
 ### Target groups `<target_group>`
 
-Target Groups collect a group of targets and specify a common label set for them.
-They are the canoncial way to specify static targets in a scrape config.
+A `target_group` allows specifying a list of targets and a common label set for them.
+They are the canoncial way to specify static targets in a scrape configuration.
 
 ```
 # The targets specified by the target group.
@@ -148,35 +149,36 @@ number.
 
 ### DNS-SD configurations `<dns_sd_config>`
 
-A DNS-SD configuration allows to specify a set of hosts for which DNS SRV records are
-queried. The DNS servers to be contacted are read from `/etc/resolv.conf`.
+A DNS-SD configuration allows specifying a set of DNS SRV record names which
+are periodically queried to discover a list of targets (host-port pairs). The
+DNS servers to be contacted are read from `/etc/resolv.conf`.
 
-The label `__meta_dns_srv_name` is attached to discovered targets with the queried
-SRV name as its value.
+During the [relabeling phase](#relabeling-relabel_config), the meta label `__meta_dns_srv_name` is
+available on each target and is set to the SRV record name that produced the
+discovered target.
 
 ```
-# A list of host names to be queried.
+# A list of DNS SRV record names to be queried.
 names:
-  [ - <host> ]
+  [ - <record_name> ]
 
 # The time after which the provided names are refreshed.
 [ refresh_interval: <duration> | default = 30s ]
 ```
 
-Where `<host>` is a valid hostname.
-
+Where `<record_name>` is any DNS SRV record name.
 
 ### Consul SD configurations `<consul_sd_config>`
 
-Consul SD configurations allow to retrieve scrape targets from [Consul's](https://www.consul.io)
+Consul SD configurations allow retrieving scrape targets from [Consul's](https://www.consul.io)
 Catalog API.
 
-The following labels are attached to targets:
+The following meta labels are available on targets during relabeling:
 
 * `__meta_consul_node`: the node name defined for the target
 * `__meta_consul_tags`: the list of tags of the target joined by the tag separator
 * `__meta_consul_service`: the name of the service the target belongs to
-* `__meta_consul_dc`: the datacenter string for the target
+* `__meta_consul_dc`: the datacenter name for the target
 
 ```
 # The information to access the Consul API. It is to be defined
@@ -192,19 +194,19 @@ server: <host>
 services:
   [ - <string> ]
 
-# The string by which consul tags are joined into the tag label.
+# The string by which Consul tags are joined into the tag label.
 [ tag_separator: <string> | default = , ]
 ```
 
 
-### File based SD configurations `<file_sd_config>`
+### File-based SD configurations `<file_sd_config>`
 
-File based service discovery provides a more dynamic way to configure static targets
+File-based service discovery provides a more generic way to configure static targets
 and serves as an interface to plug in custom service discovery mechanisms.
 
 It reads a set of files containing a list of zero or more `<target_group>`s. Changes to
 all defined files are detected via disk watches and applied immediately. Files may be
-provided in YAML or JSON format. Only changes resulting in well-formatted target groups
+provided in YAML or JSON format. Only changes resulting in well-formed target groups
 are applied.
 
 The JSON version of a target group has the following format:
@@ -218,10 +220,11 @@ The JSON version of a target group has the following format:
 }
 ```
 
-As a fallback, the file contents are re-read in the specified refresh interval.
+As a fallback, the file contents are also re-read periodically at the specified
+refresh interval.
 
-Each target has the `__meta_filepath` label attached. Its value is set to the filepath 
-from which the target was extracted.
+Each target has a meta label `__meta_filepath` during the [relabeling phase](#relabeling-relabel_config).
+Its value is set to the filepath from which the target was extracted.
 
 ```
 # Patterns for files from which target groups are extracted.
@@ -239,23 +242,25 @@ may contain a single `*` that matches any character sequence, e.g. `my/path/tg_*
 ### Relabeling `<relabel_config>`
 
 Relabeling is a powerful tool to dynamically rewrite the label set of a target before
-its gets scraped. Multiple relabeling steps can be configured per scrape config. 
-They are applied to the label set of each target in order of their configuration.
-
-Initially, aside from the configured labels, the `job` label is set to the `job_name` value
-of the surrounding scrape configuration. The `__address__` label is set to the `<host>:<port>`
-value of the target.
-After relabeling the `instance` label is set to the value of `__address__` by default if
+it gets scraped. Multiple relabeling steps can be configured per scrape configuration.
+They are applied to the label set of each target in order of their appearance
+in the configuration file.
+
+Initially, aside from the configured global and per-target labels, a target's `job`
+label is set to the `job_name` value of the respective scrape configuration.
+The `__address__` label is set to the `<host>:<port>` address of the target.
+After relabeling, the `instance` label is set to the value of `__address__` by default if
 it was not set during relabeling.
 
-Additional labels prefixed with `__meta_` may be available for relabeling. They are set
-by the service discovery mechanism that provided the target and vary between mechanisms.
+Additional labels prefixed with `__meta_` may be available during the
+relabeling phase. They are set by the service discovery mechanism that provided
+the target and vary between mechanisms.
 
 Labels starting with `__` will be removed from the label set after relabeling is completed.
 
 ```
 # The source labels select values from existing labels. Their content is concatenated
-# by the configured separator and matched against the configured regular expression.
+# using the configured separator and matched against the configured regular expression.
 source_labels: '[' <labelname> [, ...] ']'
 
 # Separator placed between concatenated source label values.
@@ -276,5 +281,12 @@ regex: <regex>
 [ action: <relabel_action> | default = replace ]
 ```
 
-Where  `<relabel_action> = drop | keep | replace` and `<regex>` is a valid
-regular expression.
+`<regex>` is any valid [RE2 regular expression](https://github.com/google/re2/wiki/Syntax).
+
+`<relabel_action>` determines the relabeling action to take:
+
+* `replace`: Match `regex` against the concatenated `source_labels`. Then, set
+  `target_label` to `replacement`, with match group references
+  (`${1}`, `${2}`, ...) in `replacement` substituted by their value.
+* `keep`: Drop targets for which `regex` does not match the concatenated `source_labels`.
+* `drop`: Drop targets for which `regex` matches the concatenated `source_labels`.