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+# Upgrade from 0.9 to 1.x
+
+## Objectives
+
+This major upgrade achieves the following objectives:
+
+1. Follow [client conventions and best practices](https://prometheus.io/docs/instrumenting/writing_clientlibs/)
+2. Add the notion of Pluggable backends. Client should be configurable with different backends: thread-safe (default), thread-unsafe (lock-free for performance on single-threaded cases), multiprocess, etc.
+Consumers should be able to build and plug their own backends based on their use cases.
+
+## Ruby
+
+The minimum supported Ruby version is 2.0.0.
+
+## Data Stores
+
+You can specify the data store implementation depending on your needs.
+
+For example, if you are running a pre-fork application using Unicorn you will need a way to aggregate the metrics from each of your workers before Prometheus scrapes them.
+
+This can be achieved with DirectFileStore.
+
+```ruby
+Prometheus::Client.config.data_store = Prometheus::Client::DataStores::DirectFileStore.new(dir: '/tmp/direct_file_store')
+```
+
+## kwargs
+
+Certain parameters are now keyword arguments.
+
+### 0.9
+```ruby
+counter = Prometheus::Client::Counter.new(:service_requests_total, '...')
+```
+
+### 1.x
+```ruby
+counter = Prometheus::Client::Counter.new(:service_requests_total, docstring: '...')
+```
+
+### Labels
+
+Labels are set when the metric is defined as opposed to when it is first used.
+
+### 0.9
+
+```ruby
+counter = Prometheus::Client::Counter.new(:service_requests_total, '...')
+counter.increment({ service: 'foo' })
+```
+
+### 1.x
+
+```ruby
+counter = Prometheus::Client::Counter.new(:service_requests_total, docstring: '...', labels: [:service])
+counter.increment(labels: { service: 'foo' })
+```
+
+## Histograms
+
+Keys from the get method are now strings.
+
+Histograms now include a "+Inf" bucket as well as the sum of all observations.
+
+### 0.9
+
+```ruby
+histogram = Prometheus::Client::Histogram.new(:service_latency_seconds, '...', {}, [0.1, 0.3, 1.2])
+
+histogram.observe({ service: 'users' }, 0.1)
+histogram.observe({ service: 'users' }, 0.3)
+histogram.observe({ service: 'users' }, 0.4)
+histogram.observe({ service: 'users' }, 1.2)
+histogram.observe({ service: 'users' }, 1.5)
+
+histogram.get({ service: 'users' })
+# => {0.1=>1.0, 0.3=>2.0, 1.2=>4.0}
+```
+### 1.x
+
+```ruby
+histogram = Prometheus::Client::Histogram.new(:service_latency_seconds, docstring: '...', labels: [:service], buckets: [0.1, 0.3, 1.2])
+
+histogram.observe(0.1, labels: { service: 'users' })
+histogram.observe(0.3, labels: { service: 'users' })
+histogram.observe(0.4, labels: { service: 'users' })
+histogram.observe(1.2, labels: { service: 'users' })
+histogram.observe(1.5, labels: { service: 'users' })
+
+histogram.get(labels: { service: 'users' })
+# => {"0.1"=>0.0, "0.3"=>1.0, "1.2"=>3.0, "+Inf"=>5.0, "sum"=>3.5}
+```
+
+## Summaries
+
+Keys from the get method are now strings.
+
+Summaries no longer include quantiles. They include the sum and the count instead.
+
+### 0.9
+
+```ruby
+summary = Prometheus::Client::Histogram.new(:service_latency_seconds, '...', {}, [0.1, 0.3, 1.2])
+
+summary.observe({ service: 'users' }, 0.1)
+summary.observe({ service: 'users' }, 0.3)
+summary.observe({ service: 'users' }, 0.4)
+summary.observe({ service: 'users' }, 1.2)
+summary.observe({ service: 'users' }, 1.5)
+
+summary.get({ service: 'users' })
+# => {0.1=>1.0, 0.3=>2.0, 1.2=>4.0}
+```
+### 1.x
+
+```ruby
+summary = Prometheus::Client::Summary.new(:service_latency_seconds, docstring: '...', labels: [:service])
+
+summary.observe(0.1, labels: { service: 'users' })
+summary.observe(0.3, labels: { service: 'users' })
+summary.observe(0.4, labels: { service: 'users' })
+summary.observe(1.2, labels: { service: 'users' })
+summary.observe(1.5, labels: { service: 'users' })
+
+summary.get(labels: { service: 'users' })
+# => {"count"=>5.0, "sum"=>3.5}
+```
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