# SimpleMemCache
Trade memory for performance.
In-memory key-value cache with expiration-time after creation or last access (a.k.a. entry time-to-live and entry idle-timeout), automatic value loading and time travel support.
## Installation
1. Add `simple_mem_cache` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[{:simple_mem_cache, "~> 1.0"}]
end
```
2. mix deps.get
```sh
$ mix deps.get
```
3. create ets table:
Only ets types: set and ordered_set are supported.
To create the ets table I recommend [Eternal](https://hex.pm/packages/eternal).
Code example:
```elixir
defmodule MyProject do
use Application
def start(_type, _args) do
import Supervisor.Spec, warn: false
Eternal.start_link(SimpleMemCache, [ :set, { :read_concurrency, true }, { :write_concurrency, true }])
```
## Usage
### Keep in cache for a limited time, automatically load new value after that
- Example: scrape news and keep it 10 minutes in cache
```elixir
us_news = SimpleMemCache.cache(SimpleMemCache, "news_us", 10, &Scraper.scrape_news_us/0)
```
- or with anonymous function:
```elixir
def news(country) do
SimpleMemCache.cache(SimpleMemCache, "news_" <> country, 10, fn -> Scraper.scrape_news(country) end)
end
```
Note about automatically new value loading:
- How long does this function take to get the new value, and is this acceptable when the old value is expired? If it takes too long, consider to use an scheduler to regularly recalculate the new value and update the cache with that.
### Keep in cache for a limited time but extend life-time everytime it is accessed
- Example: cache http response of countries rest service for at least 20 minutes
```elixir
countries_response = SimpleMemCache.cache(SimpleMemCache, "countries_response", 20, true, fn -> HTTPoison.get! "http://restcountries.eu/rest/v1/" end)
```
### Keep as long as the ets table exists
- Example: Cache products retrieved from csv file. Not a good example, because nowadays files are stored on SSD and there will be no performance gain.
```elixir
products = SimpleMemCache.cache(SimpleMemCache, "products", fn -> "products.csv"
|> File.stream!
|> CSV.parse_stream
|> Enum.to_list
end)
```
- updates are still possible:
```elixir
SimpleMemCache.put(SimpleMemCache, "products", new_value)
```
or you can force an automatically load at first access by invalidating the cached item.
### Invalidate cached item
- Example: remove products from cache
```elixir
old_value = SimpleMemCache.remove(SimpleMemCache, "products")
```
## IEx demo
```sh
$ iex -S mix
Interactive Elixir (1.3.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> f_now = fn -> DateTime.to_string(DateTime.utc_now()) end
#Function<20.52032458/0 in :erl_eval.expr/5>
iex(2)> tid = :ets.new(__MODULE__, [:set, :public, {:read_concurrency, true}, {:write_concurrency, true}])
127009
iex(3)> SimpleMemCache.put(tid, "key1", "value1")
"value1"
iex(4)> SimpleMemCache.get(tid, "key1")
{:ok, "value1"}
iex(5)> SimpleMemCache.get!(tid, "key1")
"value1"
iex(6)> SimpleMemCache.remove(tid, "key1")
"value1"
iex(7)> SimpleMemCache.get(tid, "key1")
{:not_cached, nil}
iex(8)> SimpleMemCache.get!(tid, "key1")
nil
iex(9)> IO.puts f_now.(); SimpleMemCache.put(tid, "key1", 1, "value1"); # one minute
2016-08-03 22:42:04.410133Z
"value1"
iex(10)> IO.puts f_now.(); SimpleMemCache.get(tid, "key1")
2016-08-03 22:42:36.641060Z
{:ok, "value1"}
iex(11)> IO.puts f_now.(); SimpleMemCache.get(tid, "key1")
2016-08-03 22:43:10.278992Z
{:expired, "value1"}
iex(12)> f_new_value = fn -> IO.puts "new"; "value2" end
#Function<20.52032458/0 in :erl_eval.expr/5>
iex(13)> IO.puts f_now.(); SimpleMemCache.cache(tid, "key2", 1, f_new_value); # one minute
2016-08-03 22:45:10.551159Z
new
"value2"
iex(14)> IO.puts f_now.(); SimpleMemCache.cache(tid, "key2", 1, f_new_value); # one minute
2016-08-03 22:45:16.410884Z
"value2"
iex(15)> SimpleMemCache.get(tid, "key1")
{:not_cached, nil}
iex(16)> SimpleMemCache.put(tid, "key2", "value2_changed")
"value2_changed"
iex(17)> SimpleMemCache.get(tid, "key2")
{:ok, "value2_changed"}
iex(18)> SimpleMemCache.put(tid, "key3", %{"a" => 1, "b" => {1, 2, "whatever"}}) # put whatever you want
%{"a" => 1, "b" => {1, 2, "whatever"}}
iex(19)> SimpleMemCache.get!(tid, "key3") |> Map.get("b")
{1, 2, "whatever"}
iex(20)> SimpleMemCache.stop(tid)
:ok
iex(21)> SimpleMemCache.get!(tid, "key2")
** (ArgumentError) argument error
(stdlib) :ets.lookup(127009, "Elixir.SimpleMemCache_state")
(simple_mem_cache) lib/simple_mem_cache.ex:139: SimpleMemCache.get_cache_state!/1
(simple_mem_cache) lib/simple_mem_cache.ex:105: SimpleMemCache.get/3
(simple_mem_cache) lib/simple_mem_cache.ex:111: SimpleMemCache.get!/3
```
## License
[MIT](LICENSE)
