Add migration docs for 2.0 (#3374)

* Initial draft of migration.md * Edits. * Review feedback. * Review feedback. * Staleness link to video; add docker root example; remote config file section. * s/NB/NOTE/, remove external labels link. * More typos. * Add more details link for removed PromQL features. * s/you/your/ * Expand on prom1.8/2.0 side by side setup. * More feedback. * update links. * --query.lookback-delta flag.
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+# Prometheus 2.0 migration guide
+
+In line with our [stability promise](https://prometheus.io/blog/2016/07/18/prometheus-1-0-released/#fine-print),
+the Prometheus 2.0 release contains a number of backwards incompatible changes.
+This document offers guidance on migrating from Prometheus 1.8 to Prometheus 2.0.
+
+## Flags
+
+The format of the Prometheus command line flags have changed.  Instead of a
+single dash, all flags now use a double dash. Common flags (`--config.file`,
+`--web.listen-address` and `--web.external-url`) are still the same but beyond
+that, almost all the storage-related flags have been removed.
+
+Some notable flags which have been removed:
+- `-alertmanager.url` In Prometheus 2.0, the command line flags for configuring
+  a static Alertmanager URL have been removed.  Alertmanager must now be
+  discovered via service discovery, see [Alertmanager service discovery](#amsd).
+
+- `-log.format` In Prometheus 2.0 logs can only be streamed to standard error.
+
+- `-query.staleness-delta` has been renamed to `--query.lookback-delta`; Prometheus
+  2.0 introduces a new mechanism for handling staleness, see [staleness](querying/basics.md#staleness).
+
+- `-storage.local.*` Prometheus 2.0 introduces a new storage engine, as such all
+  flags relating to the old engine have been removed.  For information on the
+  new engine, see [Storage](#storage).
+
+- `-storage.remote.*` Prometheus 2.0 has removed the already deprecated remote
+  storage flags, and will fail to start if they are supplied.  To write to
+  InfluxDB, Graphite, or OpenTSDB use the relevant storage adapter.
+
+## Alertmanager service discovery
+
+Alertmanager service discovery was introduced in Prometheus 1.4, allowing Prometheus
+to dynamically discover Alertmanager replicas using the same mechanism as scrape
+targets.  In Prometheus 2.0, the command line flags for static Alertmanager config
+have been removed, so the following command line flag:
+
+```
+./prometheus -alertmanager.url=http://alertmanager:9093/
+```
+
+Would be replaced with the following in the `prometheus.yml` config file:
+
+```yml
+alerting:
+  alertmanagers:
+  - static_configs:
+    - targets:
+      - alertmanager:9093
+```
+
+You can also use all the usual Prothetheus service discovery integrations and
+relabeling in your Alertmanager configuration.  This snippet instructs
+Prometheus to search for Kubernetes pods, in the `default` namespace, with the
+label `name: alertmanager` and with a non-empty port.
+
+```yml
+alerting:
+  alertmanagers:
+  - kubernetes_sd_configs:
+      - role: pod
+    tls_config:
+      ca_file: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
+    bearer_token_file: /var/run/secrets/kubernetes.io/serviceaccount/token
+    relabel_configs:
+    - source_labels: [__meta_kubernetes_pod_label_name]
+      regex: alertmanager
+      action: keep
+    - source_labels: [__meta_kubernetes_namespace]
+      regex: default
+      action: keep
+    - source_labels: [__meta_kubernetes_pod_container_port_number]
+      regex:
+      action: drop
+```
+
+## Recording rules and alerts
+
+The format for configuring alerting and recording rules has been changed to YAML.
+An example of a recording rule and alert in the old format:
+
+```
+job:request_duration_seconds:histogram_quantile99 =
+  histogram_quantile(0.99, sum(rate(request_duration_seconds_bucket[1m])) by (le, job))
+
+ALERT FrontendRequestLatency
+  IF job:request_duration_seconds:histogram_quantile99{job="frontend"} > 0.1
+  FOR 5m
+  ANNOTATIONS {
+    summary = "High frontend request latency",
+  }
+```
+
+Would look like this:
+
+```yml
+groups:
+- name: example.rules
+  rules:
+  - record: job:request_duration_seconds:99percentile
+    expr: histogram_quantile(0.99, sum(rate(request_duration_seconds_bucket[1m]))
+      BY (le, job))
+  - alert: FrontendRequestLatency
+    expr: job:request_duration_seconds:99percentile{job="frontend"} > 0.1
+    for: 5m
+    annotations:
+      summary: High frontend request latency
+```
+
+To help with the change, the `promtool` tool has a mode to automate the rules conversion.  Given a `.rules` file, it will output a `.rules.yml` file in the
+new format. For example:
+
+```
+$ promtool update rules example.rules
+```
+
+## Storage
+
+The data format in Prometheus 2.0 has completely changed and is not backwards
+compatible with 1.8. To retain access to your historic monitoring data we recommend
+you run a non-scraping Prometheus 1.8.1 instance in parallel with your Prometheus 2.0
+instance, and have the new server read existing data from the old one via the
+remote write protocol.
+
+Your Prometheus 1.8 instance should be started with the following flags and an
+empty config file (`empty.yml`):
+
+```
+$ ./prometheus-1.8.1.linux-amd64/prometheus -web.listen-address ":9094" -config.file empty.yml
+```
+
+NOTE: **NOTE** If you used external labels in your Prometheus 2.0 config, they need to be
+preserved in your Prometheus 1.8 config.
+Prometheus 2.0 can then be started (on the same machine) with the following flags:
+
+```
+$ ./prometheus-2.0.0.linux-amd64/prometheus --config.file prometheus.yml
+```
+
+Where `prometheus.yml` contains the stanza:
+
+```
+remote_read:
+  - url: "http://localhost:9094/api/v1/read"
+```
+
+## PromQL
+
+The follow features have been removed from PromQL:
+
+- `drop_common_labels` function - the `without` aggregation modifier should be used
+  instead.
+- `keep_common` aggregation modifier - the `by` modifier should be used instead.
+- `count_scalar` function - use cases are better handled by `absent()` or correct
+  propagation of labels in operations.
+
+See [issue #3060](https://github.com/prometheus/prometheus/issues/3060) for more
+details.
+
+## Miscellaneous
+
+### Prometheus non-root user
+
+The Prometheus Docker image is now built to [run Prometheus
+as a non-root user](https://github.com/prometheus/prometheus/pull/2859).  If you
+want the Prometheus UI/API to listen on a low port number (say, port 80), you'll
+need to override it.  For Kubernetes, you would use the following YAML:
+
+```yml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: security-context-demo-2
+spec:
+  securityContext:
+    runAsUser: 0
+...
+```
+
+See [https://kubernetes.io/docs/tasks/configure-pod-container/security-context/](Configure a Security Context for a Pod or Container)
+for more details.
+
+If you're using Docker, then the follow snippet would be used:
+
+```
+docker run -u root -p 80:80 prom/prometheus:v2.0.0-rc.2  --web.listen-address :80
+```
+
+## Prometheus lifecycle
+
+If you use the Prometheus `/-/reload` HTTP endpoint to [automatically reload your
+Prometheus config when it changes](configuration/configuration.md),
+these endpoints are disabled by default for security reasons in Prometheus 2.0.
+To enable them, set the `--web.enable-lifecycle` flag.