Copy edit "cache key" section of readme.

Minor style changes to tests.

README.md

@@ -188,9 +188,7 @@ Durability only applies when connectors throw a recoverable exception type. If a
 Caching
 -------
 
-Caching is available at the connector level if the connector implements `CacheToggle`. Connectors typically extend `CachingConnector` which implements [PSR-6][PSR-6]-compatible caching. Porter ships with just one cache implementation, `MemoryCache`, which stores data in memory but this can be substituted for any PSR-6 cache if the connector permits it.
-
-When available, the connector caches raw responses for each unique [cache key](#cache-key). Cache keys are generated by an implementation-defined strategy or the default `JsonCacheKeyGenerator` strategy.
+Caching is available at the connector level if the connector implements `CacheToggle`. Connectors typically extend `CachingConnector` which implements [PSR-6][PSR-6]-compatible caching. Porter ships with just one cache implementation, `MemoryCache`, which stores data in memory but this can be substituted for any PSR-6 cache if the connector permits it. When available, the connector caches raw responses for each unique [cache key](#cache-keys).
 
 ### Cache advice
 
@@ -214,15 +212,23 @@ $records = $porter->import(
 );
 ```
 
-### Cache key
+### Cache keys
 
-The cache key can optionally be generated by an implementation of `CacheKeyGeneratorInterface` if the connector permits it. This implementation should provide one method `generateCacheKey` which returns a [PSR-6][PSR-6]-compatible cache key.
+The cache key is generated by a `CacheKeyGenerator` that encodes parameters to produce a unique cache key for each distinct `Connector::fetch` request. The default `JsonCacheKeyGenerator` simply JSON-encodes the parameters to create a cache key. The cache key generation strategy may be changed by calling `CachingConnector::setCacheKeyGenerator`.
 
-The default implementation `JsonCacheKeyGenerator` generates keys comprised of the source and options parameters passed to `Connector::fetch`. Options are sorted before the cache key is created so the order of options are insignificant.
+#### Writing a cache key generator
 
-#### Implementation example
+The `CacheKeyGenerator` interface defines one method with the following interface.
+
+```php
+public function generateCacheKey(string $source, array $sortedOptions) : string;
+```
+
+Implementations receive arguments similar to [connectors](#connectors), with the notable exception that the options parameter is converted to an array and sorted so that the same options specified in a different order result in the same cache key. The method must return a [PSR-6][PSR-6] compatible cache key.
+
+##### Implementation example
 
-The following example demonstrates a simple cache key generation implementation using an md5 hash of the json encoded parameters.
+The following example demonstrates cache key generation using a hash of JSON-encoded parameters.
 
 ```php
 class MyCacheKeyGenerator implements CacheKeyGenerator

src/Connector/CachingConnector.php

@@ -108,13 +108,14 @@ public function setCacheKeyGenerator(CacheKeyGenerator $cacheKeyGenerator)
      *
      * @return string
      *
-     * @throws InvalidCacheKeyException
+     * @throws InvalidCacheKeyException Cache key contains invalid data.
      */
     private function validateCacheKey($key)
     {
         if (!is_string($key)) {
-            throw new InvalidCacheKeyException('Cache key must be of type string.');
+            throw new InvalidCacheKeyException('Cache key must be a string.');
         }
+
         if (strpbrk($key, self::RESERVED_CHARACTERS) !== false) {
             throw new InvalidCacheKeyException(
                 sprintf('Cache key "%s" contains one or more reserved characters: "%s"', $key, self::RESERVED_CHARACTERS)

test/Integration/Porter/Connector/CachingConnectorTest.php

@@ -97,8 +97,6 @@ public function testCacheUsedForCacheKeyGenerator()
 
     public function testFetchThrowsInvalidCacheKeyExceptionOnNonStringCackeKey()
     {
-        $this->setExpectedException(InvalidCacheKeyException::class, 'Cache key must be of type string.');
-
         $this->connector->setCacheKeyGenerator(
             \Mockery::mock(CacheKeyGenerator::class)
                 ->shouldReceive('generateCacheKey')
@@ -107,21 +105,14 @@ public function testFetchThrowsInvalidCacheKeyExceptionOnNonStringCackeKey()
                 ->getMock()
         );
 
+        $this->setExpectedException(InvalidCacheKeyException::class, 'Cache key must be a string.');
         $this->connector->fetch('quux', $this->options);
     }
 
     public function testFetchThrowsInvalidCacheKeyExceptionOnNonPSR6CompliantCacheKey()
     {
         $cacheKey = CachingConnector::RESERVED_CHARACTERS;
 
-        $this->setExpectedException(
-            InvalidCacheKeyException::class,
-            sprintf('Cache key "%s" contains one or more reserved characters: "%s"',
-                $cacheKey,
-                CachingConnector::RESERVED_CHARACTERS
-            )
-        );
-
         $this->connector->setCacheKeyGenerator(
             \Mockery::mock(CacheKeyGenerator::class)
                 ->shouldReceive('generateCacheKey')
@@ -130,6 +121,7 @@ public function testFetchThrowsInvalidCacheKeyExceptionOnNonPSR6CompliantCacheKe
                 ->getMock()
         );
 
+        $this->setExpectedException(InvalidCacheKeyException::class, 'contains one or more reserved characters');
         $this->connector->fetch('quux', $this->options);
     }