code: function requireValidElasticsearchClient (elasticsearchClient) {
      // validate client
      this.assert(typeof elasticsearchClient === 'object', 'elasticsearch required')
      this.assert(typeof elasticsearchClient.delete === 'function', 'delete method required for elasticsearch')
      this.assert(typeof elasticsearchClient.create === 'function', 'index method required for elasticsearch')
      this.assert(typeof elasticsearchClient.search === 'function', 'search method required for elasticsearch')
      this.assert(typeof elasticsearchClient.indices === 'object', 'indices object required for elasticsearch')
      this.assert(typeof elasticsearchClient.indices.create === 'function', 'indices.create method required for elasticsearch')
      // return validated client
      return elasticsearchClient
  }
docs: @function requireValidElasticsearchClient
  
  validate elasticsearch client
  
  @param {object} elasticsearchClient
  
  @returns {object}
  
  @throws {Error}