## Writing Controllers

The `controllers` package offers a variety of helpers for writing controllers.
These operate in a similar manner as [`controller-runtime`](https://github.com/kubernetes-sigs/controller-runtime) but are *far* smaller and less abstract.

With a few exceptions, Istio controllers typically are split in two phases: construction and running.

			return n, VerifyError{
				Expected: r.checksum,
				Computed: etag,
			}
		}
	}
	return n, err
}

// ETag returns the ETag of all the content read
// so far. Reading more content changes the MD5
// checksum. Therefore, calling ETag multiple
// times may return different results.
func (r *Reader) ETag() ETag {
	sum := r.md5.Sum(nil)
	return ETag(sum)
}

### Config Ingestion

Istio reads from over 20 different resources types, and aggregates them together to build the proxy configuration. These resources can be sourced from Kubernetes (via watches), files, or over xDS; Kubernetes is by far the most common usage, though.

Primarily for historical reasons, ingestion is split into a few components.

#### ConfigStore

        this.desiredAccess = access & 0x3;
        if ( this.desiredAccess == 0x3 ) {
            this.desiredAccess = 0x2; /* Mmm, I thought 0x03 was RDWR */
        }

        // map shareAccess as far as we can
        if ( ( shareAccess & SmbConstants.FILE_SHARE_READ ) != 0 && ( shareAccess & SmbConstants.FILE_SHARE_WRITE ) != 0 ) {
            this.desiredAccess |= SHARING_DENY_NONE;
        }

import java.util.concurrent.ConcurrentSkipListMap;

/**
 * Helper classes for various benchmarks.
 *
 * @author Christopher Swenson
 */
final class BenchmarkHelpers {
  /** So far, this is the best way to test various implementations of {@link Set} subclasses. */
  public interface CollectionsImplEnum {
    <E extends Comparable<E>> Collection<E> create(Collection<E> contents);

    String name();
  }

import com.google.common.primitives.Doubles;

/**
 * A mutable object which accumulates paired double values (e.g. points on a plane) and tracks some
 * basic statistics over all the values added so far. This class is not thread safe.
 *
 * @author Pete Gillin
 * @since 20.0
 */
@J2ktIncompatible
@GwtIncompatible
@ElementTypesAreNonnullByDefault
public final class PairedStatsAccumulator {

 * amortized) constant time operations. Expected in the hashtable sense (depends on the hash
 * function doing a good job of distributing the elements to the buckets to a distribution not far
 * from uniform), and amortized since some operations can trigger a hash table resize.
 *
 * <p>As compared with {@link java.util.LinkedHashMap}, this structure places significantly reduced

     * to execute other listeners.
     *
     * What about RuntimeException? If there is a bug in set()/setValue() that produces one, it will
     * propagate, too, but only as far as AbstractFuture.executeListener, which will catch and log
     * it.
     */
    setResult(transformResult);
  }

  /** Template method for subtypes to actually run the transform. */
  @ForOverride

    private static final int BLOCK_LENGTH = 64;       //    = 512 / 8;

    /**
     * 4 32-bit words (interim result)
     */
    private int[] context = new int[4];

    /**
     * Number of bytes processed so far mod. 2 power of 64.
     */
    private long count;

    /**
     * 512 bits input buffer = 16 x 32-bit words holds until reaches 512 bits.
     */
    private byte[] buffer = new byte[BLOCK_LENGTH];

    /**

be used to select a different 'environment'.

## Installing

The new installer is intended to be modular and very explicit about what is installed. It has
far more steps than the Istio installer - but each step is smaller and focused on a specific
feature, and can be performed by different people/teams at different times.